Fix it
Why is my furnace making a banging noise?
Furnace bangs split by timing: a boom at ignition is combustion, banging as air starts moving is ductwork, and rhythmic knocking during the run is mechanical. The ignition boom is the one to take seriously.
What it usually is
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Delayed ignition — gas pooling before it lights
Dirty burners light late, igniting a small accumulation of gas with a boom that can flex the cabinet. It tends to worsen gradually, and it stresses the heat exchanger every time.
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Duct oil-canning as pressure changes
Big flat sections of sheet metal pop in and out as the blower starts and stops — a single metallic bang from the same spot, more annoyance than damage.
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Expansion ticking scaled up by long duct runs
Metal warming through tens of degrees moves audibly. A series of diminishing knocks after the burner lights, fading as temperature stabilizes, fits expansion.
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A loose blower wheel or motor mount
Rhythmic knocking that tracks blower speed — and continues in fan-only mode — is mechanical, and it will not improve on its own.
What you can check yourself
Time the bang: at ignition, at blower start, or continuous? Run the fan alone (thermostat fan ON, heat off) — noise present means blower, absent means combustion or expansion. A visibly flexing duct panel can be stiffened with a screwed-on brace.
When it’s a job for a pro
A true ignition boom means burner cleaning and a heat-exchanger inspection promptly — repeated delayed ignition is how exchangers crack. Blower-side knocks need the wheel and mounts checked before something lets go at speed.
Burner cleaning typically runs $150 to $400; blower repairs $200 to $600. Duct stiffening is often a no-charge add-on during other service.
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