AC Making Strange Noises? What Buzzing, Banging and Squealing Mean

New noises from your AC are often the first warning of a problem. Catching them early can mean a smaller repair. Here's what common sounds may indicate. This is general guidance, not a diagnosis.

Common sounds

• Buzzing or humming without the fan starting: often a failing capacitor or contactor. Turn the system off and call; running it this way can damage the motor.
• Clicking at startup that repeats: may be an electrical control or thermostat issue.
• Squealing or screeching: a worn blower motor bearing or belt, or high pressure in the compressor.
• Grinding: worn motor bearings. Turn it off.
• Banging or clanking: a loose or broken part, possibly inside the compressor, or debris hitting the fan blades.
• Hissing or bubbling: can indicate a refrigerant leak or a drain issue.
• Rattling: loose panels, screws, or sticks and leaves in the outdoor unit.
• Whooshing and steam from a heat pump in winter: usually a normal defrost cycle.

What you can safely do

1. Turn off power at the thermostat and outdoor disconnect if you hear grinding, banging or smell burning.
2. With the power off, clear visible debris from around the outdoor unit.
3. Check that access panels are secured.

Don't open electrical panels on the unit. Capacitors can hold a dangerous charge even with power off.

When to call

Call a licensed HVAC contractor for any grinding, banging, buzzing without starting, or hissing. Describe the sound and when it happens; it helps the technician narrow it down.

Safety first

Some HVAC checks are safe for any homeowner: the thermostat, the air filter, a single breaker reset, and keeping the outdoor unit clear. Others are not. Leave refrigerant, capacitors, wiring inside the unit, and anything involving gas to a licensed professional. If you ever smell gas or see smoke or sparks, leave the home and call 911 or your gas utility from a safe place.

More guides

• Why Is My AC Blowing Warm Air? 8 Causes and What to Check
• AC Repair vs. Replace: How to Decide
• How Often Should You Change Your AC Filter?
• AC Frozen? What Causes Ice on the Coil and What to Do

This article is general information, not professional advice or a diagnosis. Always consult a licensed HVAC contractor about your specific system.