Snell Heating & Air Conditioning

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What’s that Creepy Sound Coming From Your Furnace?

What’s that Creepy Sound Coming From Your Furnace?

It’s starting to look a little scary around Sterling as ghosts and bats hang from trees and jack-o-lanterns light up the night with spooky faces. But what’s more scary than the things that are lurking outside are the creepy sounds you could be hearing inside your home this time of year. As the weather gets colder, you’ll probably need to kick on your furnace at night. Instead of getting scared if you hear one of these sounds, read this so you’ll know what they mean.

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Rattling as your furnace kicks on could mean you have a cracked heat exchanger, which is the biggest cause of carbon monoxide leaks so don’t ignore it. If you hear rattling during the furnace cycle, it could just be from loose connections in your duct system or loose panels that need to be tightened. We can fix that too.


A loud boom or bang when your furnace comes on can mean a delayed ignition. The boom is caused by a small gas explosion due to gas building up in your furnace chamber. This happens because the burner is not lighting properly, possibly due to a high air-to-gas ratio, not enough gas due to low gas pressure, or the burners are clogged with debris. This sound means you need to turn off your furnace and have a Snell HVAC technician check it out.


If you hear a loud house-shaking rumbling when the furnace goes off, it’s probably oil that keeps burning in your combustion chamber after the burners shut off. It could also mean there’s a problem with your burners, flame sensor, or pilot light, so be sure to schedule a maintenance visit to prevent any problems with carbon monoxide poisoning.

Tips & Insights: How Often Will a Furnace Perform Cycles In the Winter Season?

Scrape, Scrape

A metal against metal sound means there’s probably something wrong with your blower wheel. The wheel may be loose and is hitting the side of its housing, there could be a piece in your heating system that has come loose and is stuck in the wheel, or the wheel is broken. Turn off the furnace and call us before your blower or another important component of you furnace system breaks.


If you hear this, the blower belt may have loosened or slipped off its track. You’ll need to get it fixed before so it doesn’t cause other problems.

Click, Click, Click

Clicking is normal when your furnace first turns on. It’s the sound of your ignition clicking on. But if your furnace starts to click and then the blower turns on and back off again, there could be a problem with your flame sensor. Pay attention to whether your furnace is on or off when you hear the clicking, because a constant clicking when the furnace never turns on could mean a leaking gas valve.

Don’t Be Scared To Call Snell!

Our brave HVAC technicians are here to help residents around Northern Virginia with all of their furnace concerns. So if you hear something go bump in the night and you’re not sure what it is, give us a call! In addition to furnace services, our team also offers indoor air quality products and solutions to help enhance the quality of air in the home. Schedule an appointment to receive assistance with an HVAC or air quality issue.