An air conditioner is not silent, but it should be predictable. The compressor hums. The fan turns. Air moves through the ducts. You stop noticing those sounds after a few days.
A new sound means something changed. What you hear tells you roughly what happened and how urgently it needs attention.
Rattling
What it sounds like: Loose shaking, usually from the outdoor unit.
Common causes: A screw, bolt, or panel has loosened from vibration. Debris — a stick, a palm frond, a pebble — has fallen into the condenser housing and is hitting the fan blade.
What to do: Turn off the system. Look through the top grille of the outdoor unit. If you see debris, remove it. Check that the unit's cover panels are tight. If the rattling is coming from inside the air handler indoors, the blower wheel or motor mount may be loose. That requires a technician.
Urgency: Low to moderate. A loose panel is a five-minute fix. A loose blower wheel will damage the housing if it keeps running.
Buzzing
What it sounds like: An electrical hum, louder than the normal compressor hum, often from the outdoor unit.
Common causes: A failing contactor (the relay that sends power to the compressor), a bad run capacitor, or copper refrigerant lines vibrating against the cabinet. Deteriorated rubber isolation feet under the compressor can also transmit vibration into the pad and amplify it.
What to do: If the outdoor unit is buzzing but the compressor is not starting, the capacitor or contactor has likely failed. Turn the system off and call a technician. If the unit is running normally but louder than usual, check for vibrating panels first.
Urgency: Moderate. A buzzing compressor that is not starting will not fix itself.
Screeching or squealing
What it sounds like: A high-pitched shriek, usually at startup. Metal on metal.
Common causes: A failing fan motor bearing in the outdoor condenser or indoor blower. On older belt-driven systems, a worn or slipped belt. In rare cases, high internal compressor pressure.
What to do: Turn the system off immediately. A failing bearing generates heat and can seize the motor. High compressor pressure can be dangerous. Do not restart the system until a technician has inspected it.
Urgency: High. This sound almost always means a part is actively failing.
Banging
What it sounds like: A heavy thud or knock, rhythmic or intermittent, from the outdoor unit.
Common causes: A broken or unbalanced component inside the compressor — a piston, a connecting rod, or a valve plate. A loose or broken fan blade striking the housing. If the sound is from indoors, the blower wheel may have come loose from the motor shaft.
What to do: Turn the system off. A banging compressor usually means the compressor is failing internally. Running it further causes more damage and makes the repair more expensive — or turns a compressor replacement into a full system replacement.
Urgency: High. Stop the system.
Hissing
What it sounds like: A steady hiss or whistle, like air escaping from a tire.
Common causes: A refrigerant leak at a fitting, a valve, or a pinhole in the coil. Less commonly, a leaking expansion valve or a duct leak near the air handler.
What to do: If the hissing is coming from the refrigerant lines or indoor coil area, turn the system off and call a technician. Refrigerant should be handled by a licensed professional. If the hissing is from a duct joint, it is an air leak — less urgent but worth sealing.
Urgency: High for refrigerant leaks. Low for duct leaks.
Clicking
What it sounds like: Repeated clicking from the outdoor unit, or a click followed by silence.
Common causes: A single click when the thermostat calls for cooling is normal — that is the contactor engaging. Repeated clicking with the system failing to start usually means a failing capacitor, a bad contactor, or a control board issue. The system is trying to start and failing.
What to do: If the system clicks once and runs, it is working normally. If it clicks repeatedly and does not start, turn the thermostat off and call a technician.
Urgency: Moderate. The system will not start on its own.
The short version
| Sound | Most likely cause | Turn it off? |
|---|---|---|
| Rattling | Loose part or debris | Check first, then decide |
| Buzzing | Failing contactor or capacitor | Yes |
| Screeching | Bad motor bearing | Yes, immediately |
| Banging | Broken compressor or fan component | Yes, immediately |
| Hissing | Refrigerant leak | Yes |
| Clicking (repeated) | Capacitor or contactor | Yes |
If you are not sure what the sound is, describe it when you call. "It sounds like metal on metal" or "it clicks three times and stops" tells us what to bring.
Call (727) 477-3256 — 24 hours, 7 days.