Every furnace makes some noise. A soft click when it starts, a whoosh as the burners light, and the steady hum of the blower are all normal. New sounds, or sounds that get louder, are your furnace telling you something. Here is a guide to what you are hearing.
Normal furnace sounds
- A click as the thermostat calls for heat
- The hum of the inducer fan for a few seconds before the burners light
- A soft whoosh as the burners light
- The steady sound of the blower and air moving through vents
- Light ticking or popping from ductwork as it heats up and cools down
Loud bang or boom when it starts
This is usually delayed ignition. Gas builds up for a moment before it lights, then ignites all at once. Dirty burners or a weak ignitor are common causes. It is not something to ignore, because repeated small explosions stress the heat exchanger. Turn the heat off and call.
Rattling
Often a loose access panel, screw or duct. Check that the furnace door is seated. A rattle from inside the cabinet can mean a loose blower part or motor mount. Have it checked before it gets worse.
Squealing or screeching
Usually the blower motor bearings, or the belt on older belt-drive furnaces. The inducer motor can also squeal as its bearings wear. These parts are cheaper to replace before they fail completely.
Scraping or grinding metal
This usually means the blower wheel has come loose and is hitting its housing, or the motor bearings are failing. Turn the furnace off right away and call. Running it can turn a small repair into a big one.
Loud humming or buzzing
A low hum is normal. A loud hum or buzz can come from a failing blower motor, a weak capacitor or a transformer. If the hum gets louder and the blower is slow to start, the motor may be on its way out.
Repeated clicking without heat
Clicking over and over, with no burners lighting, points to an ignition problem: a failing ignitor, a dirty flame sensor, or a gas supply issue. See furnace blowing cold air for what to check.
Whistling
Whistling usually means air is being forced through a small gap. A clogged filter, closed vents, or a return that is too small are common causes. Start by replacing the filter.
Rumbling after the burners shut off
A rumble that continues after the burners should be off can mean fuel is still burning in the chamber, often from a dirty burner or gas valve problem. Turn the furnace off and call.
Noise when the AC is running
The furnace blower also moves air for your air conditioner. If you hear humming, rattling or a loud blower in summer, the cause is usually the same blower motor, capacitor or a loose panel.
When to worry
Turn the furnace off and call if you hear banging, grinding or rumbling, if you smell burning or gas, or if your carbon monoxide alarm sounds. Read our guide to carbon monoxide and furnace safety.
Is it normal to hear my furnace running?
Yes. A furnace that you can hear humming quietly is a furnace that is working. What matters is change. If it is louder than last winter, or making a sound it never made before, have it looked at. A yearly furnace tune-up catches many worn parts before they fail.
Call North Alabama Heating & A/C at (256) 534-2941. We have served Huntsville, Madison and North Alabama since 1977, Monday to Saturday, 7 AM to 7 PM. See our furnace repair service.
Quick answers
When should I worry about furnace noise?
Worry about banging or booming at startup, scraping or grinding metal, rumbling after the burners shut off, or any noise with a burning or gas smell. Turn the furnace off and call a technician for those.
What does a blower motor sound like when it’s going out?
A failing blower motor often hums loudly, squeals or grinds. It may be slow to start, run hot, or trip a breaker. Getting it checked early can prevent a no-heat call on the coldest night.
Is it normal to hear my furnace running?
Yes. A click at startup, a whoosh as the burners light and the hum of the blower are all normal. New, louder or different sounds are worth a check.
