Strange HVAC Noises: What They Mean and When to Worry

HVAC systems make noise—that's normal. But when new sounds appear, they're often warning signs. Here's what different noises typically mean.

Sounds That Need Immediate Attention

Grinding or Metal-on-Metal This usually indicates a motor bearing failure or a fan blade hitting something. Turn off the system and call for service to prevent further damage.

Loud Banging or Booming In furnaces, this can indicate delayed ignition—gas builds up before igniting, causing a small explosion. This is a safety concern that needs immediate professional attention.

Hissing from Indoor Unit A hissing sound from your indoor unit often indicates a refrigerant leak. Refrigerant is expensive and essential for cooling, and the leak needs to be found and repaired.

Sounds to Monitor

Squealing or Screeching Usually a belt issue in older systems or a motor bearing starting to fail. Not an emergency, but schedule service soon.

Rattling Often loose panels, screws, or ductwork. Check for obvious loose components first.

Clicking at Startup Some clicking is normal as relays engage. Repeated clicking without the system starting can indicate a control board or capacitor issue.

Humming Low humming is normal. Loud humming can indicate a failing compressor or electrical issue.

Outdoor Unit Sounds

Buzzing Can indicate loose parts, debris in the unit, or electrical issues with the contactor.

Clanking Often a loose fan blade or something that fell into the unit.

Pulsating or Rhythmic Noise Sometimes normal operation, especially with two-stage compressors. If it's new, have it checked.

The Bottom Line

Any new noise deserves investigation. While some are harmless, others can indicate problems that worsen quickly if ignored. When in doubt, have a technician take a look.