Why is my furnace making a banging noise?

A bang when the furnace starts is usually delayed ignition — gas builds up before lighting — or expanding ductwork. Delayed ignition deserves a technician soon: it stresses the heat exchanger. A bang when the blower stops is almost always duct metal flexing, which is annoying but harmless.

Likely causes

• Delayed ignition — Dirty burners light late; the small explosion of accumulated gas makes the boom.
• Duct expansion ('oil canning') — Thin duct metal pops as pressure changes — noise only, no danger.
• Failing blower motor or belt — Rumbling or squealing that changes with fan speed points here.

What you can check yourself

• Note when it bangs: at ignition, during the run, or at shutdown — that timing is the diagnosis.
• Replace the filter; restricted airflow makes several noises worse.
• Smell gas or see soot around the unit? Skip DIY entirely (see below).

Typical cost

Typical repairs in Washington, DC: burner cleaning/ignition service $150–$400; blower motor $400–$800; heat exchanger issues usually mean replacement ($5,500–$10,000 installed).

When to call a pro

Gas smell, soot, a carbon monoxide alarm, or a bang at every ignition cycle — call now. Heat exchanger cracks are the serious end of this symptom.

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Data by Servik — verified local pros from official license records and public sources. Cost figures are typical ranges, not quotes; a pro's written estimate is the real number.