Why Is My Furnace Making That Noise?
Furnaces are supposed to hum quietly in the background. When yours starts banging, squealing, or rattling, it’s trying to tell you something. Here’s a quick breakdown of the most common sounds and what they usually mean.
Banging or Booming
A loud bang when the furnace kicks on is often delayed ignition. Gas builds up in the combustion chamber before it lights, and the sudden ignition makes a small explosion sound. This puts stress on the heat exchanger over time. Have an HVAC tech look at it sooner rather than later.
Squealing or Whining
A high-pitched squeal usually points to a worn blower belt or a bearing that needs lubrication. If you’re comfortable with basic maintenance, you can inspect the belt yourself — a frayed or cracked belt needs replacing. Replacement belts cost a few dollars at a hardware store.
Rattling
Rattling is often the least serious noise. Check that all your vent covers are screwed down tight and that there’s no loose sheet metal on the furnace cabinet. Sometimes a screw simply vibrates loose over time. If the rattling comes from inside the unit, though, have it checked — it could be a cracked heat exchanger, which is a safety issue.
Clicking That Doesn’t Stop
A click when the furnace starts is normal. Repeated clicking that goes on and on usually means the igniter is trying to light but can’t — a dirty flame sensor is the most common cause. Cleaning the flame sensor with fine steel wool is a quick fix most homeowners can do themselves after turning off the power to the unit.
Bottom Line
Most furnace noises have a straightforward cause. Identify the sound, match it to the list above, and you’ll know whether it’s a $5 fix or a service call.