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 Seattle, WA: 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 Seattle, WA
- Seattle Steam Co · Puget Sound Plaza, 1325 4th Ave #1440, Seattle, WA · +12066236366
- Warren Company · 500 Union St Ste 900, Seattle, WA · (206) 749-9484
- US HVAC Home Service Seattle · 300 Pine St Ste 100, Seattle, WA · (253) 793-1154
- MM Comfort Systems · 800 5th Ave 101 463, Seattle, WA · 2065429100
- Dorse and Company · 5000 1st Ave S, Seattle, WA · +12062842610
- Mighty Energy Solutions · 2910 1st Ave S Ste 201, Seattle, WA · +12068801558
- United Systems Mechanical · 1400 Airport Way S Ste 202, Seattle, WA · +12069336036
- Local Hvac Expert Seattle · 3250 Airport Way S, Seattle, WA · +12065651003
Get free quotes from Seattle, WA pros →
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.