Short answer
A loud bang at startup usually means delayed ignition from dirty burners. Squealing points to a blower motor or belt. Rattling is often a loose panel or duct, and a constant hum can be a failing motor or capacitor. Shut the furnace off and call if you hear repeated booms or smell burning.
Bang or boom at startup
Gas builds up before lighting, then ignites all at once. Dirty burners or a weak igniter cause it. Repeated booms can crack the heat exchanger.
Squealing or screeching
Usually blower motor bearings or a worn belt on older units.
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Call NowRattling
Loose access panels, screws or duct joints. Also check for debris in the blower.
Humming or buzzing
Can be a failing capacitor or motor, or a transformer under load.
Clicking without heat
Repeated clicking with no flame points to an igniter or gas valve problem.
- Note when the noise happens
- Record a short video
- Turn it off if you smell burning
Related questions
Is a noisy furnace dangerous?
Some noises are harmless. Booms and burning smells are not; shut it off.
Can I oil the blower motor?
Some older motors have oil ports; most modern ones are sealed.
Services that fix this
Furnace Repair
Gas and oil furnaces that will not heat, blow cold, short-cycle or make noise.
Furnace repair in MarlboroughFurnace Ignition & Flame Sensor Repair
Burners that will not light or shut off seconds after lighting.
Ignition repair in MarlboroughThis guide is general information for homeowners, not a diagnosis of your system. For anything involving gas, carbon monoxide, electricity or active leaks, stop and call a qualified professional.
