What should I do if my furnace bangs or booms when it starts?

What a bang or boom at startup usually means

The timing of the sound matters more than the volume. A single boom right as the burners should light usually points to delayed ignition inside the burner compartment, while a pop or bang a few seconds after warm air starts moving is more often sheet-metal ductwork flexing. A light cabinet rattle from an accessible outer panel is usually separate from combustion and often comes from vibration when the inducer or blower starts.

How to tell the likely causes apart

If the noise happens exactly at ignition and sounds deeper than a panel rattle, think burner-related and stop using the furnace until it is checked. If you hear one or two hollow thumps from walls, ceilings, or basement duct runs after airflow begins, the ducts are more likely snapping as they heat and pressurize. If the sound is scraping, grinding, or repeated banging rather than one startup boom, a motor, wheel, or mount is more likely than the burners. If there is only buzzing or humming and the furnace does not start normally, the ignition sequence may be stalling before the burners or blower fully engage.

When this symptom moves from nuisance to damage risk

A minor panel rattle or occasional duct pop can stay stable for a while, but a true ignition boom tends to get worse, not better. Repeated delayed ignition can scorch burner parts, stress the heat exchanger area, and in some cases damage the burner crossover path or ignition components. Scraping and grinding are also not wait-and-see noises because a loose wheel or failing bearing can turn a small repair into a motor or housing replacement.

What a furnace repair visit usually includes for this symptom

A qualified technician will usually listen for when the noise occurs in the startup sequence, inspect the burner flames and ignition timing, and check the igniter, flame sensor, burner carryover, inducer, and blower for wear or looseness. They may remove and clean burners, verify gas pressure and combustion performance, tighten or reseat cabinet panels, and isolate whether the sound is coming from the furnace cabinet or the connected ductwork. For duct banging, they may recommend reinforcing or adjusting sections of metal rather than replacing furnace parts.

What affects the repair cost

Cost depends on whether the problem is ignition-related, mechanical, or just sheet metal. Typical 2025-2026 U.S. ranges are about $170-$575 for cleaning and minor burner or panel corrections, roughly $170-$575 for igniter, flame-sensing, or small inducer-related repairs, and around $500-$1,575 or more if an inducer motor, blower assembly, or larger duct modifications are needed. The price also rises if the furnace is hard to access, if multiple startup noises have the same root cause, or if damage from repeated booming has spread to surrounding components.