Record it
Before anything else, take thirty seconds of audio on a phone, standing close to whichever part is making the noise. Noises are intermittent, they change when a technician arrives, and describing a sound in words is unreliable. A recording removes all of that, and it frequently lets us bring the right part on the first visit.
The noises and what they mean
Rattling or vibration
Something loose. Panels, a fan guard, the unit on its mounts, or a refrigerant line contacting a surface it should be clear of. Mostly harmless in itself, but line vibration against a support eventually wears a hole and turns a rattle into a refrigerant leak. Worth fixing for that reason alone.
Screeching or grinding
A bearing failing, nearly always a fan motor — condenser or evaporator. Caught early it is a straightforward motor replacement. Left alone, the motor seizes, and if it is the condenser fan you then have a high-pressure problem and a compressor at risk.
Humming with nothing turning
A motor being energised and unable to start. Almost always a failed run or start capacitor, occasionally a seized bearing. Cheap part, quick fix, and leaving a motor humming under stall conditions burns it out.
Deep knocking or banging from the compressor
This is the one. Internal mechanical damage — a failed valve, a broken connecting rod, liquid refrigerant returning and being compressed where only vapour should be. Switch the system off. Running it further grinds metal debris through the circuit, which contaminates the oil and the lines and turns a compressor replacement into a much larger job involving flushing and a new filter drier at minimum.
Gurgling or hissing at the evaporator
Refrigerant flow you can hear. Some sound at the expansion valve is normal. A loud, continuous gurgle usually means the charge is low and vapour is passing where liquid should be, so this one often arrives alongside a box that is not holding temperature.
Clicking, repeated
A contactor or relay chattering, or a safety control cycling rapidly. Repeated clicking means something is trying and failing to start, over and over, which is hard on everything involved.
A loud bang on shutdown
Often refrigerant migration or a check valve. Usually not urgent, but it is worth mentioning because it sometimes indicates a system that is flooding back to the compressor on the off cycle, which damages it slowly.
Where the noise is coming from
Almost as useful as the noise itself. Stand in the box: evaporator fans and the expansion valve. Stand at the condensing unit: compressor, condenser fan, contactor. If you cannot tell, say that — and if the noise is loud enough to hear in both places, that is itself information, because it usually means something structural is transmitting vibration through the building.
The one piece of advice worth following
Noise is the only warning most refrigeration failures give before they happen. A bearing screeches for weeks before it seizes. A capacitor hums before the motor burns out. A compressor usually knocks before it dies. Almost every one of these is substantially cheaper to deal with while it is still a noise.
The exception, again, is compressor knocking. That is not an early warning, it is a late one, and the correct response is to switch it off rather than to finish the shift and see how it goes.
Questions we get asked
Which noise means stop the system immediately?
A deep metallic knocking or banging from the compressor itself. That is internal mechanical failure, and continuing to run it usually turns a compressor that might have been diagnosed into one that has destroyed itself and contaminated the system with debris.
Is some noise normal?
Yes. You should hear the compressor hum, the fans move air, and some refrigerant flow sound at the expansion valve. A quiet click as the thermostat cycles is normal too. What matters is a change in the usual sound rather than the presence of sound.
The noise only happens at startup. What is that?
Often a start capacitor or relay failing, or a compressor with internal mounts that have deteriorated so it shifts on start. Both are worth attention before they get worse, and both are cheaper to deal with early.