The short answer: most of the noises a heat pump makes in winter are normal. A whoosh or hiss, a cloud of steam, an outdoor fan that stops while the unit keeps running, a few minutes with no warm air indoors: makers describe all of these as the defrost cycle, which every air source heat pump runs in cold, damp weather. Clicks, ticks and cracks are usually plastic and metal moving as they warm and cool. The sounds that do need a technician are grinding, squealing, banging, a noise that is excessively loud or never stops, and ice that defrost never clears.
This applies to both ductless mini-splits and ducted heat pumps with an outdoor unit. Everything below is quoted from the makers' own manuals and support pages, and from guidance published on the U.S. Department of Energy's site. Where a maker says nothing about a sound, we say so rather than fill the gap.
Why winter is noisier
In heating mode, a heat pump pulls heat out of the outdoor air through the coil in the outdoor unit. Trane explains that "the outdoor coil is typically 10-20°F colder than the outdoor air temperature", so frost can form even when the air is above freezing. Trane says a unit "could easily enter the defrost cycle when it's in the low 40s outside. However, it's more common when the temperature is in the 20s or 30s."
To clear the frost, the unit reverses itself for a few minutes and sends hot refrigerant through the outdoor coil. LG describes the trigger plainly: "If there is a lot of frost on the outdoor unit, the heating capacity is degraded", so the defrosting function runs. Most of the winter sounds people ask about are this reversal starting, running and ending.
Our cold climate mini-split guide already sets out what Daikin's and Mitsubishi's technical manuals say about defrost itself. This page covers the full range of winter sounds, the warning signs and what you can change.
The sounds makers call normal
Defrost: the whoosh, the hiss and the steam
Trane says: "If you are outside near the heat pump, you may hear a swooshing sound as the refrigerant reverses course. The outdoor fan may stop. The noise level of the compressor may rise or fall during defrost mode." It adds that "you may notice a cloud of steam coming from your outdoor unit, which is the ice melting and evaporating, and is perfectly normal."
Lennox says: "You may hear a whooshing sound as the system shifts modes" and "You may see steam rising outside. Both are normal signs of the defrost process at work."
Fujitsu's AIRSTAGE troubleshooting FAQ says: "During Heating operation, a sizzling sound may be heard occasionally. This sound is produced by the Automatic Defrosting operation."
Mitsubishi Electric Trane, in the operating instructions for one of its ceiling cassette systems, lists "White smoke is discharged from the outdoor unit" as a symptom, and explains: "In the heating operation, vapor generated by the defrosting operation looks like white smoke."
LG says melted frost "becomes white water vapor/steam", and adds: "During the defrosting operation, warm air stops and cold refrigerant flows to the indoor unit, which may cause a contraction/expansion noise. This is a normal process of the defrosting operation, so rest assured, it is not a product defect."
Midea's ductless owner's manual says: "When the unit restarts in HEAT mode after defrosting, white mist may be emitted due to moisture generated from the defrosting process."
Indoors, the clue is usually silence. Fujitsu says its units "temporarily stop operating (between 7 and 15 minutes)" during defrost, with the OPERATION lamp flashing. Daikin's AURORA manual says that if "HEAT operation stops suddenly and a flowing sound is heard", the outdoor unit is defrosting. On a ducted system you may instead feel cool air from the vents: Trane says owners without backup heat "may notice cool air coming from their air vents instead of warm air, but again, that is perfectly normal", and that you may see "AUX or AUX heat on your thermostat when the heat pump is in defrost mode."
The outdoor fan stopping
Trane lists "The outdoor fan may stop" among normal defrost signs. Fujitsu says: "During Heating operation, the outdoor unit's fan may stop, and steam may be seen rising from the unit. This is due to Automatic Defrosting operation." A homeowner guide published on the Department of Energy's building codes site says that during defrost "Fans might stop or slow, and auxiliary heating could activate to keep the temperature stable."
The reversing valve
The whoosh at the start and end of defrost is the reversing valve switching the direction of refrigerant. Carrier says: "If you notice a gentle, whooshing sound coming from your heat pump reversing valve, there's no need to worry." LG describes the same moment: "As the outdoor unit enters the defrosting mode, the direction of refrigerant(gas) will be changed, thus generating noises." Daikin calls it a blowing sound: "If the outdoor temperature is low, this sound is generated when the direction of the flow of the refrigerant in the piping changes at the start of defrost operation, after HEAT operation is stopped and the room temperature is stabilized."
Refrigerant flow: gurgles, hisses and running water
These happen all year but are more noticeable in a quiet house on a winter night.
- Midea: "Low hissing sound during operation: This is normal and is caused by refrigerant gas flowing through both indoor and outdoor units." And: "Low hissing sound when the system starts, has just stopped running, or is defrosting: This noise is normal and is caused by the refrigerant gas stopping or changing direction."
- Daikin: a sound like flowing water "is generated because the refrigerant in the air conditioner is flowing." A whistling sound "is generated when refrigerant flows during defrosting operation."
- Mitsubishi Electric Trane: "The sound of water flowing is heard" is explained as "the sound of refrigerant or condensed water flowing in the unit." A hissing sound "is the sound when the flow of refrigerant inside the unit is switched." On multi-zone systems, a head that is switched off can still sound like running water, because "A small amount of refrigerant continues to flow into the indoor unit even though it is not operating."
- Fujitsu: "the sound of water flowing in the air conditioner's piping may be heard", during operation and right after the unit stops.
Clicks, ticks and cracks from expansion and contraction
- Daikin: a ticking sound "is generated when the cabinet and frame of the air conditioner slightly expand or shrink as a result of temperature changes."
- Mitsubishi Electric Trane: a cracking sound "is generated by the expansion/contraction of the intake grille, etc. due to change in temperature."
- Fujitsu: "a slight squeaking sound may be heard. This is the result of minute expansion and contraction of the front cover due to temperature changes."
- Midea: "A squeaking sound may occur after running the unit in HEAT mode due to expansion and contraction of the unit's plastic parts."
Compressor and valves starting up
Daikin says a clicking sound during operation or idle time "is generated when the refrigerant control valves or the electrical parts operate." Mitsubishi Electric Trane says a mechanical sound from the indoor unit "is the switching sound in turning on/off the fan or the compressor." Fujitsu says noise "may be particularly noticeable for about 2-3 minutes after starting operation (sound of coolant flowing)." LG says that "When the outdoor unit turns on or off, depending on the installation location, noises can be heard", and that "The closer the indoor unit is to the outdoor unit the relatively louder the noise might get."
LG also describes a sound most owners never expect: an oil recovery operation that "might generate unusual noises once every five minute for the time period of about 8 hours", which returns compressor oil from the pipes to the outdoor unit.
The sounds makers say need service
The makers draw the line in similar places.
- Grinding, squealing or banging. Lennox lists "You hear grinding, squealing, or banging noises from the unit" as a reason to call a professional.
- A reversing valve that roars or never stops. Carrier says: "if you begin to hear excessively loud or constant noises emanating from the reversing valve, it may be a cause for concern."
- Any abnormal sound, per the manual. Mitsubishi Electric Trane's operating instructions say to "stop using the air conditioner and consult your dealer" in several cases, including "When an abnormal sound is heard." Daikin's manual says to "Consult your dealer if the air conditioner or heat pump in operation generates unusual noise."
- Defrost that will not finish. Trane says to call your dealer if the unit "seems to be constantly switching into defrost mode or the defrost cycles last longer and longer with frost and ice starting to cover the unit." Lennox says "repeated long defrost cycles should be checked out by an HVAC professional." The Department of Energy hosted guide says: "If your heat pump is stuck in defrost or covered in ice, consult an HVAC professional."
- Ice, not frost. Carrier calls a light, thin layer of frost, generally under 1/4 inch, normal, but says "Ice buildup thicker than 1/4 inch, a solid coating of ice, or ice that does not clear during defrost is not normal and requires attention." Lennox: "Heavy ice buildup that covers the entire unit or blocks airflow signals a problem."
- A fan that hums but does not spin. Lennox lists a "Humming or buzzing sound with no fan movement" as a sign the fan is not operating properly.
Two more cautions from the makers. Carrier: "Never try to chip ice from the coil." Lennox: "Don't try to chip or break the ice." And if your unit simply seems louder than it used to, our page on why appliances get louder covers the dull causes the manuals list across many appliance types, such as a surface that is not level.
Placement and mounting: what reduces noise
Most of this is decided at installation, so it is worth raising with your installer before the unit goes in.
Keep it away from bedrooms. Daikin's AURORA installation manual says: "Avoid locations, such as near bedrooms, where the operating sound may cause disturbance." It also says to "Choose a place solid enough to bear the weight and vibration of the unit, where the operating sound will not be amplified." Daikin UK adds: "Avoid tight corners or enclosed areas that can reflect and amplify sound." For the rest of the room, our quiet bedroom page covers the other machines that run while you sleep.
Think about the neighbors. Mitsubishi Electric Trane's manual says to avoid installing where "the operation sound or air from the outdoor unit bothers the house next door." The Department of Energy and New Buildings Institute permitting checklist notes that "Some jurisdictions have ordinances that dictate where outdoor equipment can be installed", and that air source heat pump outdoor units "are typically much quieter than familiar air conditioning condensing units".
Isolate vibration. Daikin UK says: "Rubber pads or anti-vibration mounts prevent mechanical vibration from transferring into walls or timber structures." Fujitsu's Australian support team explains why mounting matters: "Vibration transmitted through a wall, roof, deck, bracket or pipework can sound significantly louder inside the building than it does beside the outdoor unit." It names the usual culprits: a wall bracket "Transfers compressor movement directly into the wall structure", lightweight cladding "Can act as a sounding board and amplify low-frequency vibration", and timber decks and roofs "May flex or resonate at compressor operating frequencies." The DOE checklist asks inspectors to confirm "Supports and foundations built to prevent excessive vibration, settlement or movement of the equipment." Daikin's manual also gives a suggested shortest pipe length of 10ft (3m), "in order to avoid noise from the outdoor unit and vibration."
Plan for snow. The DOE checklist says: "In locations susceptible to snow, outdoor units must be installed on stands above the snow line. A best practice is sheltering the outdoor unit from potential snow or ice that could fall on it." The DOE hosted homeowner guide says to "shield it from excessive water and snow, which can accumulate from the roof", and that "Preferred installation is on the home's gable end." Lennox says to "Ensure gutters and downspouts don't drain onto or near the heat pump."
Keep it clear. Lennox: "Clear snow away from the unit after storms, but don't pile it against the sides." Carrier: "Keep at least 24 inches of clear space around the heat pump for proper airflow, while following the clearance requirements for your specific model." Daikin's manual warns that obstacles close to the outdoor unit "may drop the performance of the outdoor unit or increase the operating sound of the outdoor unit." The DOE hosted guide says to avoid covering the unit in winter, and to make sure defrost water "does not pool and refreeze on a walkway."
How inverter units sound in the cold
Most modern mini-splits and many ducted heat pumps use an inverter compressor, which changes speed instead of switching fully on and off. Daikin UK says: "Inverter compressors reduce start-stop noise by adjusting output to match demand." The trade is that in deep cold the compressor runs faster for longer. Mitsubishi Electric says its "Variable-capacity heat pumps with Hyper-Heating INVERTER® (H2i®) technology allow for higher compressor heating speeds at lower outdoor air temperatures".
Makers describe what that means for sound in a few ways:
- Daikin's AURORA manual answers "Operating sound is loud." with: "When frost forms on the heat exchanger of the outdoor unit, the operating sound level increases slightly."
- Daikin UK says: "Icing, snow or restricted airflow can force units to run at higher capacity, raising sound output until conditions improve", and that "Any rise in sound levels is usually linked to temporary defrost cycles, after which normal operation resumes."
- Trane says the noise level of the compressor "may rise or fall during defrost mode."
- Fujitsu Australia says: "A vibration that appears only within a narrow compressor-speed range commonly indicates resonance between the outdoor unit and the supporting structure rather than an internal mechanical failure." In other words, a hum that comes and goes at one speed is often the mounting, not the machine.
This is also why the decibel figure on a spec sheet is not what you hear on a January night. Mitsubishi Electric's MSZ-GX service manual notes that its sound level is measured in an anechoic room and that "The sound may be bigger than displayed level under actual installation condition by surrounding echoes." Our explainer on comparing decibel ratings covers why figures from different makers rarely line up. If you are still choosing a system, our quietest mini-splits guide and the brand pages for Mitsubishi and Daikin list each maker's published heating figures with the fan speed they describe. For a ducted system, our quietest heat pumps guide ranks outdoor units by their AHRI 270 heating figures, size by size.
What to do, in order
- Wait and watch. If the noise comes with steam, a stopped outdoor fan or a pause in warm air indoors, it is almost certainly defrost. Note how long it lasts and compare it with the time your own manual gives.
- Look at the outdoor unit. A thin, even frost is normal. Thick ice that stays after defrost, or snow banked against the sides, is not.
- Clear it gently. Brush away snow and leaves. Do not chip ice, and do not cover the unit.
- Check what drips onto it. A gutter or roof edge dripping onto the unit can freeze it repeatedly.
- Feel for vibration. If a hum is louder indoors than beside the unit, the bracket, deck or pipework may be carrying it. That is an installer's job.
- Call for service if you hear grinding, squealing or banging, a valve noise that is excessively loud or constant, or defrost cycles that keep getting longer.
We did not measure any heat pump for this page, and we have not tested any of these fixes. Every sound and every figure here is quoted from the maker's own manual or support page, or from guidance published on the Department of Energy's site, with the conditions each one gives.