Some furnace sounds are normal: a soft click when it starts, the whoosh of burners lighting and a low hum from the blower. Loud banging at startup, high-pitched squealing, metal rattling or a constant loud hum usually point to a problem like delayed ignition, a worn motor or belt, or a loose part. Turn the furnace off and call a technician for any loud bang or scraping sound.
Banging or booming
A loud bang when the burners light is often delayed ignition: gas builds up for a moment before it lights. Dirty burners or a weak igniter can cause it. This is not something to live with, because repeated small explosions can stress the heat exchanger. Turn the thermostat down and call for service.
A pop or tick from the ductwork as the system heats up and cools down is usually metal ducts expanding and shrinking. It is annoying but usually harmless. Undersized return air or high duct pressure can make it worse, which a technician can check during a tune-up.
Squealing or screeching
- Older belt-drive blowers: a worn or loose belt squeals, especially at startup.
- Blower or inducer motor bearings: a high-pitched squeal or screech can mean a motor is failing.
- Grinding or scraping: metal-on-metal sounds mean something is rubbing. Turn the furnace off and call, because running it can cause more damage.
Rattling
A rattle can be as simple as a loose access panel or a loose screw on the duct. First, make sure the furnace door is seated firmly. If the rattle comes from inside the cabinet, it could be a loose blower wheel, a loose inducer fan or, less often, a cracked heat exchanger. Because a cracked heat exchanger can let carbon monoxide escape, have a rattling furnace inspected, and make sure your CO alarms are working.
Humming and buzzing
A low, steady hum from the blower is normal. A loud hum or buzz, especially if the blower doesn't start, can mean a failing capacitor or motor, or a transformer or relay problem. Electrical buzzing that gets louder, or any burning smell, is a reason to turn the furnace off at its power switch and call.
Clicking
One or two clicks at startup and shutdown are normal: that is the thermostat and relays. Repeated clicking with no flame usually means the furnace is trying to light and failing, such as a bad igniter, a gas supply problem or a flame sensor issue. Clicking from the thermostat itself may be a wiring or battery problem.
Quick guide: normal or not?
| Sound | Usually | What to do |
|---|---|---|
| Soft click at start and stop | Normal | Nothing |
| Short whoosh as burners light | Normal | Nothing |
| Ticking or popping in ducts | Usually harmless | Mention it at your next tune-up |
| Loud bang at ignition | Problem | Turn heat down and call |
| Squeal or screech | Worn belt or motor | Call soon |
| Grinding or scraping | Problem | Turn furnace off and call |
| Rattle inside cabinet | Loose part or worse | Check door, then call |
| Repeated clicks, no flame | Ignition problem | Call |
Before you call, note when the sound happens (startup, while running, or shutdown) and where it seems to come from. That helps a technician find the cause faster and can shorten the visit.
When to call a pro
Call right away for banging at startup, grinding, burning smells, or any noise along with a CO alarm (leave the house first and call 911 if the CO alarm sounds). Call soon for squealing, rattling or new humming. Yogi Heating & Cooling diagnoses furnace noises in Schaumburg and the northwest suburbs. Call (814) 431-1709 or book online. A yearly tune-up through our maintenance plan helps catch worn motors and dirty burners before they get loud.
More questions
Is it normal for a furnace to make noise when it turns on?
A soft click, a short whoosh as the burners light and a steady blower hum are normal. A loud bang or boom is not.
Why does my furnace bang when it shuts off?
It's often ductwork contracting as it cools. If the bang comes from inside the furnace instead, have a technician check it.
Can a noisy furnace be dangerous?
Some noises, like banging at ignition or rattling from a cracked heat exchanger, can be linked to safety problems. Keep CO alarms working and get loud new sounds checked.
Can I oil my furnace motor myself?
Most modern blower motors are sealed and don't need oil. We recommend leaving motor work inside the furnace to a technician.
Related
- Furnace and heating repair
- Maintenance plan
- What to do when your carbon monoxide alarm goes off
- How much does a new furnace cost in the Chicago suburbs?