Air conditioners are supposed to hum quietly. New noises are the system telling you what's wrong — and each sound points to a different part.
Buzzing
Usually electrical: a failing capacitor, loose wiring or a contactor that's welded closed. If the outdoor fan isn't spinning while it buzzes, shut it off.
Clicking that never turns into running
The thermostat is calling, the contactor is trying, but the unit isn't starting — often a capacitor or a locked compressor.
Grinding or screeching
Motor bearings in the condenser fan or blower. Catch it early and it's a motor; ignore it and it's a seized motor plus a fried capacitor.
Hissing or bubbling
Refrigerant leaking. Hissing at the indoor coil or the line set means a leak repair is needed before recharging.
Banging or clanking
A broken fan blade, a loose mount, or — inside the compressor — internal damage. See AC compressor repair.
Rattling
Loose panel screws or debris in the condenser. Easy fix, but check it before the fan blade hits something.
Any noise paired with warm air or a tripping breaker is worth a same-day call for AC repair.