Furnace starts making a noise you have not heard before. You wake up to it at 2am in January. Furnace noise is one of the most common winter calls we handle across the GTA. Here is the diagnostic our techs run on the phone before they dispatch, with the noises that are dangerous and the ones that are just annoying.
Stop and Call Us If You Hear Any of These Right Now
- Rotten egg or sulphur smell (gas leak): do not touch switches, breakers or the thermostat, and do not use a phone inside. Leave the house immediately and call Enbridge Gas at 1-866-763-5427 or 911 from outside
- Loud rolling boom with black soot near the furnace (cracked heat exchanger)
- Persistent loud banging on every startup (delayed ignition, fire risk)
- Hissing sound near a gas line (gas leak): leave and call Enbridge or 911 from outside, as above
Noise Diagnostic Table
| What you hear | Likely cause | Severity | Typical fix |
|---|---|---|---|
| Banging on every startup | Delayed ignition: gas pools then ignites all at once | Urgent | Burner cleaning, gas valve adjustment, $250 to $450 |
| Rolling boom plus soot | Cracked heat exchanger | Critical | Stop using immediately, replace furnace |
| Continuous rumbling | Dirty burners or oversized unit | Medium | Burner cleaning $200 to $300 |
| Screeching or squealing | Failed blower motor bearing or belt | Medium | Motor or belt replacement $300 to $700 |
| Rapid clicking from control board | Failed relay or transformer | Medium | Control board $400 to $750 |
| High-pitched whistling | Ductwork leak or filter restriction | Low | Replace filter, seal ducts $0 to $300 |
| Steady humming or buzzing | Transformer or capacitor wearing out | Low to Medium | Component swap $150 to $300 |
| Pinging from ducts when blower starts | Sheet metal expanding under air flow | Low | Often cosmetic, sometimes duct stiffener $100 to $200 |
| Fan never shuts off | Thermostat or fan relay stuck | Low to Medium | Reset thermostat or replace relay $150 to $250 |
How to Shut Off Your Furnace Safely
If you smell gas (rotten eggs or sulphur), skip these steps. Do not flip any switch, breaker or thermostat, and do not use a phone inside. Leave the house immediately and call Enbridge Gas at 1-866-763-5427 or 911 from outside. For a noise with no gas smell:
- Turn the thermostat to OFF first so the furnace stops calling for heat
- Locate the gas shutoff valve on the gas line directly above or beside the furnace, then turn it 90 degrees so the handle sits across the pipe
- Flip the furnace breaker at the electrical panel to OFF
- If you smell smoke or burning, leave the house and call 911 from outside
- Call EcoFrost at (416) 835-4775 to book the repair
Why Banging on Ignition Is Always Urgent
Banging on ignition means gas is pooling in the burner area before the igniter fires. When it finally lights, the accumulated gas combusts in a small explosion. Repeated ignition bangs can crack the heat exchanger, which then leaks carbon monoxide into the home. Our techs treat every banging-on-ignition call as urgent.
How EcoFrost Diagnoses Over the Phone
When you call (416) 835-4775, our dispatch tech asks four questions: when did the noise start, does it happen on startup or all the time, can you see flame colour and is it blue or yellow, and is there any smell. From those answers we know whether to send a same-day truck or whether you should shut the furnace off and book first thing in the morning.
Need a same-day furnace repair across Mississauga, Toronto and the GTA? Call us now.
Call (416) 835-4775Or see our furnace repair pricing and process for your city.
Furnace Repair Pricing?Frequently Asked Questions
My furnace just started banging on startup. Is it dangerous?
How much does it cost to fix a noisy furnace in Mississauga?
Can I keep using my furnace if it makes a loud humming sound?
Why does my furnace ductwork sound like a pop can when the blower starts?
When should I shut off the gas to my furnace?
EcoFrost Heating & Cooling
Toronto's Trusted HVAC Experts Since 2015
Our certified HVAC technicians have completed 15,000+ installs across the Toronto area. We write about heating, cooling, and air quality from real field experience not marketing copy. Learn about us →
