HVAC systems make normal operating sounds. But new, unusual, or loud noises often signal problems. Here is what different sounds typically indicate.
Banging or Popping
From ductwork: Metal ducts expand when heated and contract when cooling stops. Some popping is normal. Excessive noise may indicate undersized ducts or poor support.
From furnace: Banging at startup could indicate delayed ignition where gas builds up before igniting. This is potentially dangerous and needs immediate professional attention.
From AC: A single bang when the compressor starts or stops is often normal. Repeated banging during operation may indicate a loose or damaged component.
Squealing or Screeching
Belt-driven systems: Worn or slipping belts squeal, especially at startup. Belt replacement is straightforward maintenance.
Motor bearings: Worn bearings in blower or fan motors squeal during operation. Without attention, the motor will eventually fail.
Compressor: Squealing from the outdoor unit may indicate high internal pressure or failing compressor. Turn off the system and call for service.
Grinding
Metal-on-metal grinding sounds require immediate attention. Something is rubbing against something else and causing damage with every rotation.
Turn off the system and call for service before further damage occurs.
Rattling or Vibrating
Loose panels: Check furnace and air handler access panels. Loose screws or latches cause rattling during operation.
Debris: Something may be caught in the blower or outdoor fan. Turn off the system and check for foreign objects.
Ductwork: Loose duct connections rattle when air flows through.
Clicking
At startup and shutdown: Normal for many systems as relays engage and disengage.
Continuous clicking: May indicate a failing relay, contactor, or control board. Clicking without the system starting suggests an electrical problem.
Hissing
Near refrigerant lines: A refrigerant leak may cause hissing. This needs professional repair.
Near ducts: Escaping air from duct leaks can sound like hissing.
From the gas furnace: Could indicate a gas leak. If you smell gas, leave immediately and call your gas utility.
When to Act
New sounds that were not present before usually indicate something has changed. Address unusual noises promptly before minor issues become major repairs.
Immediate attention: Grinding, loud banging from furnace, squealing from compressor, hissing with gas smell
Prompt attention: Squealing from motors, rattling, continuous clicking
Monitor: Duct popping, startup clicks