A healthy air conditioner runs with a steady, low hum. So when your AC suddenly starts buzzing, banging, or screeching, it's trying to tell you something. Here's how to decode the most common sounds — and which ones mean "call a pro now."
Buzzing
A buzzing sound often points to an electrical issue — a failing capacitor, loose wiring, or a contactor relay going bad. It can also be a struggling compressor. Because electrical faults can be a safety risk, buzzing is worth a professional look.
Rattling or vibrating
Rattling usually means something is loose — a panel, screws, or debris like twigs and leaves in the outdoor unit. Sometimes it's a failing fan motor. Turn the unit off and check for visible debris; if the rattle continues, have it inspected.
Clicking
A single click at startup and shutdown is normal. Constant or rapid clicking usually signals a failing thermostat or a defective relay. If your AC clicks but won't start, see our guide on what to do when your AC won't turn on.
Screeching or squealing
A high-pitched screech often means a bad fan motor bearing or a worn belt in older systems. It can also indicate dangerously high pressure in the compressor. Don't ignore this one.
Banging or clanking
Loud banging usually points to a loose or broken part inside the compressor or a disconnected component — typically a repair, or on an older unit, a replacement conversation.
Hissing or bubbling
Hissing can mean a refrigerant leak, which also makes your AC blow warm. Learn the signs in our post on AC refrigerant leaks.
When to call a pro
Loose debris you can safely clear yourself is fine to handle. But buzzing, screeching, banging, and hissing all point to internal or electrical problems that need a technician. Thermo Master offers fast AC repair across Palatine and the NW suburbs — reach out and we'll track down the noise before it becomes a breakdown.