Buzzing, clicking, grinding, hissing: what each AC noise usually means

Steady buzzing from the outdoor unit with the fan still is the classic failed capacitor. Buzzing that comes and goes with the unit cycling is more often the contactor, the switch that sends power to the compressor. Buzzing when the thermostat is not calling means the contactor is stuck closed, which runs the compressor nonstop and needs attention the same day. A faint buzz from the indoor unit's transformer is normal.

One click when the system starts and one when it stops is the contactor doing its job. Rapid or repeated clicking without the system starting is a relay, a control board or a thermostat with weak batteries trying and failing to call. Change the thermostat batteries first; it is the cheapest fix on this page.

Bearings. Either the condenser fan motor outside or the blower motor inside. A squeal that rises and falls with speed is the same thing earlier in its life. Turn the system off: a motor that seizes can take the capacitor and, in the case of the blower, overheat the furnace. Motor replacement is a few hundred dollars; letting it seize on a 105-degree day adds the wait.

At the outdoor unit or along the copper lines, hissing is refrigerant escaping and the system should be turned off until a technician finds the leak. At a supply register or in the attic, hissing is more often a duct leak, which wastes money rather than equipment. A steady loud hiss or a bubbling sound at the indoor coil is a larger leak and the system should not run.

A bang when the compressor starts can be a compressor with internal damage or a hard-start problem, and a rattle from the outdoor unit is usually a loose panel or debris in the fan. Loose panels are a homeowner fix; anything from inside the compressor is a professional diagnosis and often a repair-or-replace conversation.

Gurgling at the indoor unit is water in a partly blocked drain line or, less often, refrigerant moving through a low charge. Dripping over a window or eave is the attic overflow pan's secondary drain telling you the primary line is clogged. Both are in our drain line guide.