Heat pumps make more kinds of noise than air conditioners, because they run in both seasons and they switch modes. Several sounds that alarm people are completely normal. A few mean a part is failing. Here is how to tell them apart.
Normal heat pump sounds
- A loud whoosh, then silence, then the fan restarting. That is the reversing valve shifting at the start or end of a defrost cycle. On a cold morning you may hear it and see steam rising off the outdoor unit. Completely normal.
- Steam and dripping water outdoors in winter. The unit melting frost off its coil. Also normal.
- A soft hum while running. The compressor and fan motors at work.
- A click at startup and shutdown. The contactor or a relay switching.
- Gentle gurgling. Refrigerant moving through the lines, more noticeable right after a mode change.
Sounds that need attention
Loud buzzing from the outdoor unit
A steady buzz when the unit should be running usually points to an electrical part. A failing contactor buzzes as its contacts chatter. A weak capacitor makes the compressor or fan motor hum without starting. Loose wiring can buzz. If the fan is not spinning while the unit buzzes, turn the system off, because a compressor that cannot start overheats quickly.
Humming with no fan movement
The classic failed capacitor or seized fan motor. An inexpensive part and a short visit when caught early. Do not keep switching it back on.
Vibrating or rattling
Often the least serious. Loose panel screws, a fan guard, debris in the housing, or the unit sitting unevenly on its pad after soil settles. It can also be a line set vibrating against the house wall. If the rattle comes with a new change in how the unit sounds under load, it may be the compressor mounts.
Grinding or screeching
Metal on metal from a fan or blower motor with failing bearings. Motors in this state usually stop completely within weeks, and in Sacramento that tends to happen on the hottest or coldest day of the season.
Rapid clicking that repeats
The system trying to start over and over. Often a thermostat, control board or capacitor problem. Repeated failed starts are hard on the compressor.
Hissing that continues
A brief hiss during defrost is normal. A continuous hiss from the lines or the unit can mean a refrigerant leak. Refrigerant is not used up, so a system that is losing it is leaking. Our article on refrigerant costs explains why that repair matters more now.
Loud banging
Something loose inside the cabinet being hit by the fan, or a compressor knocking on failed mounts. Turn the system off at the disconnect beside the unit.
Indoor noises
From the air handler, the common ones are a whistle from a clogged filter or undersized return, a rattle from a loose panel, and a squeal from a blower motor. A loud hum or buzz at the air handler in winter can be the electric backup heat strips coming on, which is normal in small doses. If they seem to run constantly, the heat pump itself may not be keeping up.
How to describe it when you call
- Indoor or outdoor
- Heating mode, cooling mode, or both
- At startup, while running, or at shutdown
- Whether the house is still heating or cooling normally
A ten second phone recording is the most useful thing you can send.
What we do
We locate the noise, measure current draw on the motors, test the capacitor and contactor under load, and check the mounts and fan assembly. Most noise calls end with a capacitor, a motor or loose hardware. Our diagnostic is a flat $69 and it is credited toward the repair.
Related: AC making a loud noise, furnace noises, heat pump not turning on, and our heat pump services page.
Frequently asked questions
Is it normal for a heat pump to make a whooshing noise?
Yes. A loud whoosh followed by a pause is usually the reversing valve shifting at the start or end of a defrost cycle. It is often accompanied by steam from the outdoor unit on cold mornings.
Why is my heat pump buzzing?
A loud buzz from the outdoor unit usually points to an electrical part such as a failing contactor, a weak capacitor or loose wiring. If the fan is not spinning, turn the system off.
Why is my heat pump vibrating?
Loose panels or fan guards, debris in the housing, an uneven pad or a line set touching the wall are the common causes. Vibration that comes with a change in the running sound can point to the compressor mounts.
Should a heat pump make noise in defrost mode?
Yes. Defrost involves the reversing valve shifting and the outdoor fan stopping, which produces a whoosh and sometimes a loud hiss. Steam and dripping water are also normal.
Why does my heat pump hum but not turn on?
The motor has power but cannot start. A failed run capacitor or a seized fan motor is the usual cause. Leaving it on risks overheating the compressor.
Can I record the noise for the technician?
Please do. A short recording often lets us bring the right part on the first visit.