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 Austin, TX: 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 Austin, TX
- 24/7 AC Repair Austin · 704 San Antonio St., Austin, TX · 7372154475
- Classic Aire Mechanical, Inc. · 1108 Lavaca St Ste 110-487, Austin, TX · +15124478300
- Tex-air Heating & Air Conditioning · 506 Congress Ave, Austin, TX · +15125637505
- Gold Eagle Services · 501 Congress Ave Ste 150, Austin, TX · 8303023864
- RedHome HVAC Services · 823 Congress Ave Ste 150, Austin, TX · 5124027277
- Quick Cooling Inc. · 500 E 4th St #323, Austin, TX · 5129192193
- Hot Rod Mechanical · 3415 E 5th St, Austin, TX · +15123868686
- Green Leaf A/C and Heating · 1405 Clifford Ave, Austin, TX · 5126404790
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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.