Heat pumps run far more hours than an AC ever did — heating and cooling, all year — so owners hear more of their repertoire and worry more about it. A good share of "strange noise" calls end with us confirming the sound is part of the design. Here's the by-ear field guide from thousands of driveway listening sessions.
Sounds that are just heat pump life
The defrost whoosh-hiss: every so often in cold weather the unit pauses, makes a pronounced whoosh (the reversing valve flipping), hisses, steams dramatically, then resumes. Pure design. Gurgling and soft boiling in the lines at start and stop — refrigerant equalizing. A deeper hum on frigid mornings as the compressor works harder, and on inverter models a rising-falling pitch as the unit modulates — that variable drone is efficiency, not distress.
Sounds with a work order attached
Metallic scraping or ticking in time with the fan: blades hitting ice (see our icing guide) or a fan going off-center — shut it down before blades meet coil. A rising scream or dry screech: fan motor bearings dying; the motor is replaceable now, or the motor-plus-cooked-capacitor is replaceable later. Hard buzzing with no start, or rapid clicking: the capacitor/contactor duo auditioning for our other articles. A knock or rattle from deep in the compressor at start/stop that's grown over months: internal mounts or valves wearing — this one wants a professional ear soon, because compressors telegraph before they fail. Panel buzz and cabinet rattle: truly trivial — loose screws and vibration — but worth fixing because a panel vibrating against a line set can wear a refrigerant leak over years.
The one worth a same-day call
A loud mechanical noise followed by silence — the unit protected itself and stopped. Don't cycle it repeatedly to "see if it clears"; whatever stopped it is still in there, and restarts under fault are how small failures recruit expensive ones.
Our approach
Describe the sound on the phone (or hold the phone near the unit while it does it — that really helps) and we'll often narrow it before arriving. On site: listen, isolate with the fan and compressor separated, meter the suspects, written quote. Most noise repairs are motors, mounts and hardware — the affordable end of heat pump work when caught at the noise stage rather than the silence stage.