Quick answer
A boom at startup is delayed ignition from dirty burners; banging from the ducts is sheet metal expanding. Squealing means a blower belt or a motor bearing running dry, grinding means the blower wheel or bearings have failed, and rapid clicking is an igniter or flame sensor failing to prove flame. Shut the furnace off for grinding or repeated booms and book a licensed technician.
A furnace making a loud banging or squealing noise is telling you which part is wearing out, if you know how to listen. Every furnace makes some noise — an inducer whirring, a blower ramping up, ducts ticking as they warm — but a new sound, or one that gets louder over a few weeks, has a specific mechanical cause. The list below matches each noise to its part, then explains which sounds justify shutting the furnace down until someone looks at it.
Decode the noise
- Boom or bang at startup — delayed ignition. Gas pools in the burner box for a moment before it lights, then ignites all at once. The usual cause is rust and scale on the burners or in the crossover ports, sometimes a weak igniter. Each boom flexes the heat exchanger.
- Pinging or banging from the ductwork — thermal expansion of sheet metal, or an undersized return duct 'oil-canning' when the blower starts and the duct wall pops inward. Annoying, not dangerous.
- High-pitched squeal — on a belt-drive furnace from the 1980s or earlier, a glazed or slipping belt; on anything newer, blower or inducer motor bearings running dry.
- Grinding, metal on metal — a blower wheel that has loosened on its shaft and is hitting the housing, or a motor whose bearings have collapsed. Shut it off; running it can destroy the motor and the wheel together.
- Rapid clicking — the igniter and gas valve cycling as the furnace tries to light and the flame sensor fails to prove flame. Usually a $150–$450 repair.
- Rumbling after the burners shut off — burners not shutting down cleanly, dirty burners, or a gas valve leaking through. Worth a professional look soon.
- Whistling — air pulled through a gap: a badly seated filter, a filter that is too restrictive, or a return-duct leak near the furnace.
- A loud hum with no airflow — a blower motor trying to start on a failed capacitor. It will overheat and trip its thermal protector if left.
Noises that mean: turn it off now
Two sounds justify switching the furnace off at the service switch and calling for emergency furnace service. The first is repeated booming at ignition: each event stresses the heat exchanger, and a cracked exchanger can let combustion gases into the airstream. The second is metallic grinding, because a blower that seizes can burn out its motor and, on some furnaces, take the control board with it. If you ever smell gas alongside any noise, leave the house and call Enbridge Gas at 1-866-763-5427 from outside before you call anyone else.
Furnace noise or duct noise?
Stand beside the furnace with the blower door on. If the sound comes from inside the cabinet and changes when the blower speeds up or the burners cut in, it is the furnace. If it comes from the ductwork overhead a few seconds after the blower starts and stops when it does, it is the ducts. Duct banging is common in 1970s–90s subdivision homes where a larger, higher-airflow furnace was later dropped onto the original return trunk; the fix is a bigger return or a lower blower speed, not a furnace repair. A duct that booms once at each start and stop can usually be stiffened with a cross-break or a brace.
Why furnace noises get expensive when ignored
Most furnace noises start as sub-$300 repairs. A squealing bearing caught early is a lubrication or motor-mount fix; ignored for a season, it becomes a $500–$1,500 blower motor replacement. Delayed ignition caught early is a burner cleaning; ignored, it can crack the heat exchanger on a furnace past 15 years, at which point you are reading the furnace replacement warning signs instead of a repair quote. A clicking igniter that never lights is also the start of short-cycling, which wears every other part in the ignition sequence.
What a noisy-furnace repair costs in the GTA
- Diagnostic visit: $90–$150, folded into the repair if you go ahead
- Burner cleaning and igniter check for delayed ignition: covered by a $120–$250 tune-up if nothing has broken
- Flame sensor cleaning or replacement: $150–$300
- Hot surface igniter: $200–$450
- Draft inducer motor — whining or grinding from the small fan at the top of the furnace: $400–$900
- Blower motor: $500–$1,500, PSC at the low end and variable-speed ECM at the top
An annual furnace tune-up includes burner cleaning, blower inspection and motor amp checks that head off nearly every sound on this list. If your furnace is making a noise it did not make last month, ZK Mechanical's licensed technicians cover the whole GTA and Hamilton — book a diagnostic or call (647) 801-1252.