Normal sounds people worry about
- Gurgling / flowing water - refrigerant moving between units, loudest at startup, mode changes, and defrost. Normal.
- Crackling / ticking plastic - the housing expanding and contracting with temperature swings. Normal.
- Whoosh or hiss for a few minutes in heating - defrost cycle reversing flow. Normal (steam outside confirms it).
- Clicks at start/stop - relays and the vane motors homing. Normal.
Safe checks first
- Rattles and buzzes: find the loose thing. Front panel not fully latched, filter not seated, or items on top of the unit vibrate loudest. Press the panel edges while it runs - if the sound changes, reseat the panel and filters.
- Outdoor unit vibration. Loose mounting-bracket bolts or a pad walking out of level turns normal compressor hum into a wall-borne drone. Check visually; snug obvious loose bolts on the mounts (unit OFF). Rubber isolation pads under the feet are a cheap upgrade worth asking your installer about.
- Debris check. A leaf or twig ticking in the outdoor fan is common after storms - power OFF at the disconnect, look, remove what's reachable without disassembly.
- New noise after filter cleaning? A filter reinstalled off its track flutters loudly at high fan. Reseat.
Beyond the safe checks (technician scope)
Grinding, screeching, or metal-on-metal (fan motor bearings - indoor or outdoor), a compressor that has grown markedly louder or knocks at start, and any noise paired with error codes or performance loss. Bearing noises worsen; run time makes the repair bigger.
When to stop and call
Grinding or screeching anywhere (stop running it), knocking from the outdoor unit at startup, humming that never becomes a start (see outdoor unit not running), or any burning smell with the noise - breaker off first.