What's normal
These sounds are part of a normal heating cycle:
- A click or two at the start and end of a cycle from relays and the gas valve.
- The hum of the small inducer fan running briefly before the burners light.
- A soft whoosh when the burners light, then the main blower starting a short time later.
- Light ticking or popping from metal ducts as they warm up and cool down.
A bang or boom when the heat starts
A bang or small boom right as the burners light is often delayed ignition: gas builds up for a moment before it lights. Dirty burners, a weak igniter or a gas pressure problem can cause it. It puts stress on the heat exchanger, so turn the heat off at the thermostat and call.
A single pop from the ductwork a minute or so after the heat starts or stops is different. It's usually metal ducts flexing as they heat and cool. It can be annoying, but it usually isn't dangerous.
Squealing or screeching
A high-pitched squeal usually comes from a worn motor bearing in the blower or the inducer fan, or from a worn belt on older furnaces that have one. Bearings tend to get louder and can eventually seize, so have it checked before it stops the furnace.
Rattling or vibrating
Rattling is often a loose access panel, screw or duct strap, or something loose in the blower. Make sure the furnace door is seated correctly. If the rattle comes from inside the furnace or keeps getting worse, turn the heat off and call.
Clicking over and over with no heat
One click is normal. Clicking again and again without the burners lighting usually means the furnace is trying to light and failing, often because of a worn igniter, a dirty flame sensor or a gas supply problem. After a few tries most furnaces lock out. Don't keep resetting it; call for a diagnosis.
Grinding, scraping or a loud hum
Grinding or scraping metal can mean the blower wheel has come loose or a motor bearing has failed. Turn the system off to prevent more damage. A loud hum with no air coming out of the vents often points to a blower motor or capacitor that has failed.
Whistling
Whistling usually means air is being forced through a small opening: a clogged filter, closed or blocked vents, or a gap in the ductwork. Replace the filter and open the vents first.
Gurgling on a high-efficiency furnace
High-efficiency furnaces make water as they run and drain it away. Gurgling usually means the condensate drain or trap is partly clogged. If the water backs up, the furnace can shut itself off.
When to turn it off and call
Turn the heat off at the thermostat and call if you notice any of these:
- A bang or boom when the burners light
- Grinding or scraping metal
- A burning or electrical smell
- Clicking over and over with no flame
- Any noise along with a carbon monoxide alarm. Get everyone outside and call 911 first.
How HVAC Doctor can help
Call or text (859) 684-3259 and one of the owners, Ahmad or Ammar, answers, day or night. Mention our online special and the diagnostic is $39 (regularly $79), credited to the repair, and you get a firm price before any work starts. See our furnace repair page for what we fix.
Common questions
Why does my furnace bang when it turns on?
A bang right as the burners light is often delayed ignition, where gas builds up briefly before it lights. Dirty burners, a weak igniter or a gas pressure problem are common causes. Turn the heat off and have it checked, because repeated delayed ignition stresses the heat exchanger.
Is it normal for ducts to pop when the heat comes on?
Yes. A pop or tick from metal ducts as they warm up and cool down is common and usually harmless. A bang from the furnace itself at the moment the burners light is different and should be checked.
Why is my furnace squealing?
Usually a worn motor bearing in the blower or inducer fan, or a worn belt on an older furnace. It tends to get worse, so have it checked before the motor fails.
Why does my furnace keep clicking but not light?
It's trying to light and failing. Common causes are a worn igniter, a dirty flame sensor or a gas supply problem. Don't keep resetting it; call for a diagnosis.
Can I keep running a noisy furnace?
A little more noise than usual can usually wait for a scheduled visit. A bang when the burners light, grinding, a burning smell or clicking with no heat are reasons to shut it off and call now.