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 Miami, FL: 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 Miami, FL
- New Duct Service Company · 1851 Delaware Pkwy, Miami, FL · +15612698633
- AC Repair Miami Pros · 3580 NW 54th St, Miami, FL · +17866864127
- Barrios Air Conditioning Inc · 3461 NW 48th St, Miami, FL · +13056370109
- Tri-County Mechanical Inc · 3305 NW 37th St, Miami, FL · 3052561858
- Ice Team Corporation · 3274 NW 38th St, Miami, FL · 17862566371
- Og Ac Distributor · 3235 NW 37th St, Miami, FL · +13056335144
- Navas Multiservices Corporation · 3050 NW 40th St, Miami, FL · 13053251077
- Nicot Air Conditioning Inc · 2385 Nw 70th Ave Unit A02, Miami, FL · 17862831366
Get free quotes from Miami, FL 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.