Does this match your problem?
- Loud bang or boom on startup
- High-pitched squeal from the blower area
- Repeated clicking with no ignition
- Grinding, scraping, or rattling metal
- Continuous rumbling after burners shut off
Quick check first
If you smell gas OR hear a boom on ignition, shut the system off at the furnace switch and call a pro. Do not keep running it.
Step-by-step troubleshooting
Bang or boom on ignition
Delayed ignition, gas builds up in the burner chamber before it lights, then ignites all at once. Caused by dirty burners, weak igniter, or low gas pressure. This can crack the heat exchanger. Shut it off and call a technician.
Warning: A cracked heat exchanger leaks carbon monoxide. Do not keep running a furnace that booms on startup.
High-pitched squeal
Usually a worn blower belt or dry motor bearings. Newer direct-drive furnaces don't have belts, so the noise points to a failing blower motor or inducer motor. Not immediately dangerous, but the part will fail soon, schedule service.
Rapid clicking without ignition
The igniter is trying but the burners aren't lighting. Causes: no gas (check the valve), failed hot-surface igniter, or dirty flame sensor. Modern furnaces will lock out after 3 tries. Safe to investigate, cheap to fix.
Grinding or scraping metal
Blower wheel is loose on its shaft or the bearings are shot. Turn the furnace OFF at the switch immediately, running it will destroy the motor and possibly the wheel. Call a pro.
Rumbling after shutdown
Burners aren't extinguishing cleanly, usually from dirty burners or poor combustion. Not an emergency, but schedule a combustion tune-up, dirty burners produce more carbon monoxide.
Popping or ticking in ducts
Sheet-metal ducts expand and contract with temperature. This is normal and does not require service unless it's brand new and dramatically loud.
Stop and call a licensed pro if:
- •Any boom, bang, or explosive sound on ignition
- •Grinding or metal-on-metal scraping
- •The noise is paired with a burning smell or visible smoke
- •You have any carbon monoxide alarm activity
- •The noise started right after a recent repair