What that furnace noise means
Banging, screeching, clicking, rumbling. A rough guide to which ones can wait until morning and which one means switch it off now.
Ryan DeRue · September 14, 2026 · 1 min read
Furnaces make noise. Some of it is normal, some of it is a warning, and one of them means stop. Here is the shortlist.
A loud bang a few seconds after it starts
Usually delayed ignition: gas building up before it lights, then igniting all at once. It can also be ductwork flexing under pressure, which is harmless. Delayed ignition is not harmless and it wants looking at soon.
Screeching or squealing
Blower motor bearings or a slipping belt on older units. Not dangerous, but it gets worse and it will eventually stop the blower, which stops the heat.
Rumbling that continues after the heat stops
Often burners that need cleaning, or a pilot out of adjustment. Worth a service call rather than an emergency one.
Rapid clicking with no ignition
The igniter is trying and failing. Usually a failed igniter or flame sensor, and both are quick, inexpensive parts.
Scraping metal on metal
Switch it off. That is usually the blower wheel hitting its housing, and running it does real damage.
The one that is not a noise
If you hear any of the above and also smell gas, or a carbon monoxide alarm sounds, leave the house first and call from outside. Nothing on this page is worth staying indoors to diagnose.
Read it and still not sure?
Describe what the system is doing and we will tell you whether it is a five minute fix, a real repair, or a replacement conversation. That costs nothing.
Quick answers
Is a booming noise on startup dangerous?
It can be. A bang a few seconds after the furnace starts is often delayed ignition, where gas accumulates before lighting. It stresses the heat exchanger and it is worth having looked at rather than living with.