Furnace Making Loud Noise

A delayed ignition thump or a scraping blower means a part is already working past what it was built for, so call before it lets go in the middle of a cold night.

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When this becomes an emergency

There's a thump or a whoomp from the cabinet a second or two after the burner tries to light.
A high squeal rises with the blower and fades when the fan winds down.
Something is scraping inside the blower housing, metal against metal.
The ductwork booms as it expands and the noise carries through the whole house.

What a pro does

Sound is a diagnostic in its own right, and the timing of a noise narrows things down faster than the noise itself. A thump a second or two after the burner tries to light is unburned fuel gathering before it catches, which puts a shock load on the exchanger every single time. Techs listen for when in the cycle it happens. Scraping usually means a blower wheel has shifted on its shaft or the bearings have gone dry, and it's worth shutting the system down at the breaker rather than letting the wheel chew into the housing overnight. Booming ductwork in an older house with wood heat alongside the furnace is often nothing more than thin sheet metal oil-canning as it warms, which is annoying and harmless. Not everything that's loud is broken. Sorting the harmless ones from the ones that matter is most of the visit.

Frequently asked questions

How do you tell duct noise from furnace noise?

Where you hear it, and when. Duct booming travels along the runs and lands hardest in the rooms farthest out, while a mechanical fault stays in the cabinet and tracks the blower or the burner.

Is a squeal that only happens on startup a problem?

It's usually a bearing or a belt that needs attention. It won't strand you tonight, though a squeal that gets longer over a few weeks tends to end in a dead blower motor.

Should the furnace be shut off because of a noise?

For anything that sounds like metal grinding on metal, yes, cut it at the breaker. A soft rattle or a boom in the ducts can wait for a normal appointment.

Does a noisy furnace in an unheated part of the house matter more?

It gets noticed less, which is the risk. A blower failing in a back utility room or a crawlspace can run itself to death before anyone in the living room hears anything wrong.

Furnace Making Loud Noise?

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