Quick Answer
A single boom at startup usually means delayed gas ignition; metallic banging from ducts is normal expansion. Squealing points to a worn blower belt or failing motor bearing, grinding means a motor problem that needs immediate attention, and rapid clicking is often a failing igniter or flame sensor. Turn the furnace off for grinding or repeated booming and call a licensed technician.
Furnaces are never silent, but new noises — especially ones that get louder over weeks — are the system telling you a component is wearing out. Identifying the sound narrows the cause considerably. Here's what each noise typically means in a gas furnace, and which ones justify shutting the system down.
Decode the noise
- Boom or bang at startup — delayed ignition: gas builds up before lighting, then ignites all at once. Often caused by dirty burners. This stresses the heat exchanger and should be serviced promptly.
- Pinging/banging from ductwork — thermal expansion and contraction of sheet metal, or undersized ducts "oil-canning" when the blower starts. Annoying but harmless.
- High-pitched squeal — blower belt slipping (older furnaces) or motor bearings running dry. Serviceable cheaply if caught early.
- Grinding metal-on-metal — blower wheel loose on its shaft or bearings gone. Shut the furnace off; running it can destroy the motor.
- Rapid clicking — igniter trying and failing to light, or a flame sensor losing the flame signal. Usually a $150–$450 repair.
- Rumbling after the burners shut off — dirty burners or a flame issue worth a professional look.
- Whistling — air being pulled through gaps, often simply a badly seated filter or return-air leak.
Noises that mean: turn it off now
Two sounds warrant switching the furnace off at the service switch and calling for emergency furnace service: repeated booming at ignition (each event micro-stresses the heat exchanger, and a cracked heat exchanger can leak combustion gases) and metallic grinding (a failing blower can seize and burn out). If you ever smell gas alongside any noise, leave the house and call Enbridge's emergency line before calling anyone else.
Why 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 — at which point you're reading our furnace replacement guide instead.
An annual furnace tune-up includes burner cleaning, blower inspection, and motor checks that prevent nearly all of these noises. If your furnace is making any sound it didn't make last month, ZK Mechanical's licensed technicians serve the entire GTA — book a diagnostic or call (647) 801-1252.
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