Here’s What You Should Do If Your Furnace is Making Strange Noises

noise

We’re well into winter and your gas or electric furnace has been running for several months now. If you’ve started to notice strange noises coming from the system or your home’s air ducts, it could be a sign that your furnace is on the verge of a breakdown. In this article, we’ll review some common sounds furnaces make, how to distinguish normal sounds from noises that should cause concern, and when you need to call in a professional for an expert assessment of your furnace.

Rule out normal operating sounds

All HVAC systems make some degree of noise while running. But, how do you distinguish between a “normal” sound from something more sinister? Start by listening and ruling out several of these common furnace sounds:

  • Light Booms: As the hot air generated by the furnace enters the much colder ducts, the now-heated metal expands. When the furnace stops, the metal contracts again. The result can be a low booming sound as the metal “warps” back into place.
  • Humming: All gas furnaces make noise as they start up and run. A gentle hum or very minor rattle is typically nothing to be concerned about. When your furnace first starts up, you might hear the hum of gas running, followed by a “click” of ignition.
  • Rumbling: Many furnaces rumble as they operate. This could be a sign that some of your ducts are loose but is typically not a warning sign of an impending repair need.

What sounds may indicate a malfunction?

Here are just some of the noises that may indicate that your gas or electric furnace has an issue. Remember: always have a certified and experienced HVAC technician work on your system. Trying to diagnose and fix problems yourself could lead to damage to the unit and a voided manufacturer’s warranty.

  • Metal Scraping: If you’re hearing a high-pitched squeal or metal scraping sound coming from the furnace unit, turn the system off and call in a local pro. A likely culprit is a loose blower wheel that is now scraping against its casing. Beyond this noise being incredibly annoying, ignoring this problem could lead to further damage in the blower wheel assembly.
  • Loud Banging: This is distinct from the air duct contraction-expansion booms mentioned earlier. If this sound is loud and coming from inside the furnace itself, it could indicate that the burners are dirty. As they become covered in dust and grime, the burners are unable to quickly combust all the gas entering the chamber, leading to gas buildup and—essentially—a miniature explosion that you hear in the form of quick “pop” or loud “bang.”
  • Screeching: Often, this indicates that something is off about the belt assembly. Or, it could be a bad bearing in the motor. In either case, you’ll need a local HVAC expert to come out and inspect the system. Furnace belts can deteriorate or become loose. When this happens, they need to be either replaced or put back in place. With the bearings, a special type of lubricant can reduce the noise they create while the system is operational.

When in doubt, call a professional

As seen with “light booms” and “loud banging,” discerning harmless furnace noises from dangerous ones can be a difficult task for any homeowner. We recommend you lean on the side of caution. When in doubt, always have a locally trusted HVAC company out to inspect your system and diagnose any strange noises. 

Unusual furnace noises are often the metaphorical “canary in the coal mine” for your system, providing you with early warning of an impending breakdown. The sooner you address such issues, the less damage they’ll do to the furnace long-term.

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