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 New York, NY: 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.
HVAC pros in New York, NY
- Ptac Inc · 38 Park Row, New York, NY · 16316415210
- Best Cool Air New York · 72 Nassau St 2nd Floor, New York, NY · 7182851414
- Lacor Mechanical Systems · 114 Fulton St Rm 2e, New York, NY · +12122335607
- PTAC Air Conditioning Services · 42 Fulton St, New York, NY · +12129207080
- Aled Technologies · 233 Water St, New York, NY · 19178638286
- Jameson and Sons Cooling Air Experts · 26 Broadway, New York, NY · +16466049204
- Up Heating Cooling & PTAC Services · 376 E 77th St, New York, NY · +15168546003
- Cool Air Inc Manhattan - HVAC Contractor | PTAC Services | AC Repair and Cleaning · 509 E 78th St #1b, New York, NY · +17187212131
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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.