Symptoms
- Noise coming from the furnace/air handler cabinet or ductwork when the AC runs
- May be constant or only during startup/shutdown
- Airflow may or may not be affected
Likely causes
- CommonLoose access panel or screws vibratingPanels can loosen over time from normal blower vibration.
- CommonDirty or unbalanced blower wheelDust buildup on the blower blades can throw it off balance and cause noise.
- PossibleWorn blower motor bearingsAging bearings can produce a squeal or grind as the motor spins.
- PossibleLoose or misaligned blower belt (belt-drive systems)An older belt-drive blower can squeal if the belt is loose, worn, or misaligned.
- Less commonDebris in the ductwork or blower housingAn object that fell into the system can rattle or bang around.
What to do
- Turn off the system and check that all visible access panels are firmly latched or screwed in place.
- Replace the air filter, since a very dirty one can sometimes cause the blower to work harder and vibrate more.
- Listen closely to identify whether the noise is a rattle (loose part), squeal (belt or bearing), or thud (unbalanced blower or debris).
- If accessible and you're comfortable, visually check the blower wheel for heavy dust buildup that might be causing imbalance.
- Don't attempt to open the blower compartment or touch moving parts while the system is running.
- Call an HVAC technician for anything beyond a loose panel, especially squealing, grinding, or persistent rattling.
Tools and parts
- Screwdriver
When to call a pro
Call an HVAC technician for squealing, grinding, or persistent rattling that isn't fixed by tightening an accessible panel.
Safety
Use caution
Never reach into the blower compartment while the system is running.