Strange HVAC Sounds: What They Mean and What to Do

Banging or Clanking

What it means: A loose or broken component — often a connecting rod, piston pin, or crankshaft inside the compressor. Also possible: loose blower fan blades or debris inside the unit.

What to do: Shut the system off immediately. Operating a system with loose internal components accelerates damage. Call for service.

Squealing or Screeching

What it means: Worn or failing blower motor bearings, a slipping or broken belt (older belt-drive systems), or refrigerant pressure issues.

What to do: Motor bearing issues worsen rapidly. Schedule service promptly to avoid complete motor failure.

Hissing

What it means: Refrigerant leaking from the system (higher-pitch hiss), or a leaking duct connection (lower-pitch whooshing sound). A loud hiss from the furnace may indicate a cracked heat exchanger.

What to do: High-pressure refrigerant leaks require immediate attention. Duct leaks reduce efficiency significantly.

Rattling

What it means: Loose panels, screws, or debris (leaves, sticks) inside the outdoor unit. Also possible: loose ductwork connections vibrating with airflow.

What to do: Check for visible debris around the outdoor unit. Tighten loose access panels. If the rattling is inside the unit, call for an inspection.

Humming or Buzzing

What it means: Electrical issues — loose wiring, failing contactor, or a capacitor beginning to fail. A buzzing outdoor unit that doesn't start often points to a bad capacitor.

What to do: Don't ignore electrical sounds. Call for service — capacitor replacement is inexpensive; ignoring it can lead to compressor failure.

Clicking (Repeated)

What it means: The igniter is attempting to light but failing (furnace), or electrical relay issues. A few clicks at startup is normal — repeated clicking indicates a problem.