Most heat pump outdoor unit noise is completely normal. These machines contain a compressor, a fan, an expansion device and a valve that reverses the direction of refrigerant flow — all of that makes sound, and a unit running silently would be a unit that is not running.
Three sounds are different, and two of them get worse the longer you leave them. This page maps each noise to the part making it, and gives you a verdict rather than a suggestion to book an inspection. If the sound is coming from the head inside the room rather than the box outside, that is a different problem — most often biofilm on the blower wheel.
First: is the noise new?
This matters more than which noise it is. A unit that has hummed and clunked the same way through six winters is telling you nothing. A sound that appeared last week is data.
Before you diagnose anything, answer three questions: Is it new? Does it change with the fan speed? Does it happen at startup, at shutdown, or continuously? Those three answers narrow it faster than any description of the sound itself.
If you can, record it on your phone from a fixed spot. Phone microphones are poor at absolute volume but very good at telling you whether next month sounds worse than this month.
Heat pump outdoor unit noise, sound by sound
| Sound | Usually is | Verdict | What to do |
|---|---|---|---|
| Steady low hum while running | Compressor and fan, working | Normal | Nothing |
| Gentle whoosh at start and stop | Refrigerant pressures equalising | Normal | Nothing |
| Single loud clunk when the mode changes | Reversing valve switching | Normal | Nothing |
| Whoosh, steam and a bang in cold weather | Defrost cycle ending | Normal | Nothing — this alarms people every winter |
| Pulsing that rises and falls | Inverter compressor modulating | Normal | Nothing |
| Click at start and stop | Contactor closing | Normal | Nothing |
| Rapid repeated clicking | Short cycling, or a failing contactor | Book it | Note how often it cycles, then call |
| Rattle that changes with fan speed | Loose panel, screws, or debris | DIY | Power off, open, look |
| Hum you feel through the floor indoors | Vibration transmitted by the mounts | Book it | Isolation pads or bracket, not a volume problem |
| Gurgling | Condensate drain, or low charge | Book it | Check the drain first |
| Screeching or squealing | Fan motor bearings | Stop | Switch off at the breaker |
| Grinding | Bearings, or the blade striking something | Stop | Switch off at the breaker |
| Loud buzz with the fan not turning | Failed capacitor or stalled motor | Stop | Breaker off now |
| Continuous hiss | Refrigerant escaping | Stop | Certified technician only |
The three that mean stop now
Everything else on that list can wait for an appointment. These three cannot, and the reason is the same in each case: continuing to run the unit converts a moderate repair into a replacement.
- A buzz with the fan not turning. The motor is receiving power and not moving — usually a dead run capacitor. A stalled motor draws heavy current and cooks its own windings within minutes. Kill the breaker. This is the single most expensive noise to ignore.
- Grinding or screeching from the fan. Bearings are failing. Running it grinds them until the blade shifts and contacts the housing, which turns a motor swap into a motor and coil repair.
- A hiss that does not stop. Refrigerant is escaping. Beyond the environmental issue, running a system low on charge overheats the compressor, and the compressor is the most expensive part in the box.
The alarming noises that are completely normal
Three sounds generate more unnecessary call-outs than anything else on a heat pump.
The winter bang and cloud of steam
In heating mode the outdoor coil ices up, and periodically the unit reverses itself to melt it. You hear a loud clunk, the fan stops, steam rises off the unit, and a few minutes later it clunks again and resumes. That is the defrost cycle finishing, and it is designed behaviour. It is worth knowing what icing looks like when it is a problem, which is a different article.
The clunk when heating turns to cooling
The reversing valve is a solenoid moving a slide inside a pressurised pipe. It is not subtle. One firm clunk when the mode changes is the valve doing its job.
Sound that rises and falls without you touching anything
An inverter-driven compressor runs at whatever speed the load needs rather than simply switching on and off. The pitch drifting up and down is the system modulating, and it is the reason inverter units are quieter overall than the fixed-speed ones they replaced.
What you can fix yourself
Rattles are the one category worth attacking without a technician, and they are common. Isolate power at the breaker first — not the thermostat.
- Panel screws. Vibration works them loose over a few seasons. Go round every one with a nut driver.
- Debris in the fan guard. Leaves, twigs, a stone thrown up by a mower. Look before you assume the worst; this is the single most common cause of a sudden new rattle.
- The service valve caps and line insulation. Loose caps buzz, and degraded insulation lets pipes touch the casing and drum.
- Clearance. A unit too close to a wall or fence reflects its own noise back and sounds far louder than it is. If yours was installed tight to something, that is a placement problem — the same logic that governs where an indoor head goes applies outside.
What you should not do: spray anything into the fan while it runs, remove the electrical panel cover, or attempt to replace a capacitor. Capacitors hold a charge after the power is off.
Noise, neighbours and the rules
A modern residential outdoor unit typically sits somewhere around 50–60 dB(A) at one metre — roughly conversation level, and well below anything with an occupational health implication of the kind OSHA regulates. The complaints that arise are almost never about loudness in absolute terms.
They are about three things instead: night-time operation, when ambient noise drops and a steady hum becomes obvious; tonal character, because a constant single-pitch hum is far more irritating than louder broadband noise; and proximity to a boundary or bedroom window.
Limits are set locally and vary enormously, often expressed as a maximum at the neighbouring property line with a lower figure at night. If you are planning an installation rather than reacting to a complaint, check the rule before you choose the position, not after.
When to call someone
- Any of the three stop-now noises.
- A rattle you cannot find after checking panels and debris.
- Rapid repeated clicking, which usually means the system is short cycling — a control or charge problem, not a noise problem.
- Vibration felt inside the house, which needs the mounting arrangement changed rather than the unit repaired.
- Any new noise accompanied by weaker heating or cooling. Two symptoms together is worth a diagnostic visit; either alone often is not. Our other troubleshooting guides cover the symptoms that show up indoors.
Worth saying plainly: a noise that is on the normal list, has not changed, and comes with a system that heats and cools properly does not need a service call, whatever a search result tells you.
Frequently asked questions
My heat pump is louder in winter than in summer. Is that a fault?
Usually not. Heating mode works the compressor harder, defrost cycles add their own noises, and cold still air carries sound further than warm air. Louder in winter is expected; differently loud is what to watch.
Can I build a box around the unit to quiet it?
Not a sealed one. The unit needs a large volume of air through the coil, and restricting it raises running cost, cuts capacity and can damage the compressor. An open-sided acoustic screen that leaves the required clearances is the safe version; check the clearances in your manual before building anything.
Is a buzzing sound always electrical?
No, but a buzz that starts and stops exactly with the unit and comes from the panel area usually is. The one that matters is a buzz with the fan stationary. That combination is not ambiguous.
The unit hisses for about thirty seconds after it shuts down. Should I worry?
No. That is pressure equalising between the high and low sides, and it stops on its own. A hiss that continues indefinitely is the one to act on.
How loud should a new unit be?
Manufacturers publish a sound power or sound pressure rating on the specification sheet, usually between the high forties and low sixties dB(A) for residential equipment. It is one of the few specifications worth comparing directly between models if the unit will sit near a bedroom.
Found something wrong here?
Corrections are welcome in the comments below and they get acted on. If you work in the trade and something here is inaccurate, incomplete, or true in your region but not in ours, please say so — publicly is fine, and better than privately. This site does not yet carry review by a certified technician, which means a practitioner disagreeing in the open is the closest thing it has to peer review. Comments are held for approval before they appear, so there is a short delay and no spam.
Scope note. Checking panels, clearances and debris with the power isolated is homeowner work. Anything inside the electrical compartment is not: capacitors store a dangerous charge after disconnection. Anything involving refrigerant requires certification — in the United States that is EPA Section 608, with equivalent schemes in the UK, EU and Australia. Always isolate at the breaker, never at the thermostat.