Quick answer
A heat pump making a loud noise is normal when it is a whoosh and hiss lasting a few seconds, the reversing valve shifting at the start and end of defrost, or a steady drone from the inverter compressor in deep cold. Clacking, grinding, buzzing, rattling or a bang at start-up is not. Shut the unit down for any metal-on-metal sound or burning smell.
Why your heat pump is making a loud noise comes down to what kind of sound it is, how long it lasts and when it happens. A heat pump is louder in winter than an air conditioner ever is in summer, and two of its loudest sounds are normal. The ones that matter are new, rhythmic or mechanical. Listen from a few metres away for a full minute before deciding which you have.
Normal: the defrost whoosh and a compressor working hard in the cold
The loudest normal sound is defrost. To melt frost off the outdoor coil, the reversing valve swaps the high- and low-pressure sides of the system, and the pressures equalising make a sudden whoosh followed by a hiss. The outdoor fan stops, steam rises, and a few minutes later a second whoosh marks the switch back to heating. It lasts seconds, it repeats more often in damp weather near freezing, and it needs no action.
The second is the compressor itself. An inverter compressor changes speed to match the load, and as the temperature falls toward -15 °C it ramps up and holds a higher, steadier drone for hours. Gurgling in the lines, ticking as metal panels shrink in the cold and a brief hum at start-up belong on the normal list too. For sound ratings and placement, see how loud a heat pump is.
Abnormal sounds and what each one points to
- Rhythmic clacking or thwacking: fan blades striking ice that has built up in the base or on the coil. Shut it off; a chipped blade throws the fan out of balance and ruins the motor bearings.
- Rattling: a loose cabinet panel, a missing screw or a refrigerant line touching the casing. Press on each panel while it runs to find the one that goes quiet.
- Grinding or squealing: worn fan-motor bearings. The noise comes and goes at first, then becomes constant, and the motor seizes soon after.
- Buzzing from a conventional unit: a contactor with pitted contacts or a weak coil chatters instead of closing cleanly. A loud hum with nothing turning is more often the capacitor.
- Knock or bang as the compressor starts: hard starting, liquid refrigerant reaching the compressor, or broken mounts inside the shell. A single clunk at shutdown is ordinary; a bang at every start is not.
- Loud hiss that does not stop: a refrigerant leak, not the few seconds of a defrost changeover.
Vibration carried through wall brackets and line-sets
A hum you hear inside the house but barely notice outside is vibration, not airborne noise. A unit on a wall bracket passes compressor vibration into the framing, and a wood-frame wall behaves like a drum skin, loudest in the room on the other side. The refrigerant line-set does the same wherever it is strapped tight to a joist or pinched in the wall sleeve.
The fixes are mechanical and usually modest: rubber isolation mounts under the feet, cushioned clamps on the line-set, a loop of slack where the lines leave the unit, or moving the unit from the bracket to a ground stand. Rubber feet harden after years of sun and cold and stop absorbing anything, so a unit that grew louder gradually often just needs new ones. In newer subdivisions with narrow side yards, the unit sits between two walls that reflect sound back and forth, which makes a small vibration fault seem bigger.
Check this yourself first, and when to shut the heat pump down
- Time it. A sound that lasts under a minute and repeats every hour or so in cold, damp weather is defrost.
- With the power off, look through the grille for ice, a stick or a toy in the fan's path.
- Check that the unit sits level and all of its feet rest on the stand or pad.
- Press on the panels and the line-set cover to find a rattle.
- Note the outdoor temperature. Louder at -15 °C than at +5 °C is expected from an inverter unit.
- Shut down and call now for metal-on-metal scraping, grinding or screeching.
- Shut down and call now if fan blades are hitting ice or the fan guard.
- Shut down and call now for a burning or hot-electrical smell at the outdoor unit or the air handler.
- Shut down and call now after a bang followed by the breaker tripping.
- Shut down and call now for buzzing with the fan and compressor both stopped.
To shut down, set the thermostat to EM HEAT so the backup carries the house, then pull the disconnect or switch off the breaker for the outdoor unit. Do not restart it to let someone hear the noise; a short phone video is enough.
What heat pump noise repairs cost
- Diagnostic: $90–$150, deducted from the repair.
- Contactor or capacitor: $150–$350.
- Outdoor fan motor: $300–$700.
- Most other noise repairs: $150–$800, with compressor work and relocation to a stand quoted in writing.
Send the video with your booking and say whether the noise is constant, at start-up or only in defrost; that alone often identifies the part. ZK Mechanical repairs conventional and cold-climate heat pumps across Halton, Hamilton and Mississauga. Go to heat pump repair, compare notes with why a mini-split makes noise if yours is ductless, browse the heat pump topic page, or contact us.