Just about every household furnace makes a few characteristic sounds. In most cases, annual furnace tuneups can keep the racket to a minimum. But if you're noticing these unusual noises from your furnace or ductwork, it's a sign your heating system needs help.
Squealing from the furnace
Just like a squeaky wheel, your furnace's blower motor may start to squeal if it needs lubricant. The blower motor is what sends the hot air from the furnace into the rest of the home. You can add lubricant yourself to stop the noise. Be sure to check your furnace's user guide to make sure you use the right type of lube. However, sometimes a squealing or screeching sound is caused by a motor blower belt that's slipped out of place, or problems with the motor bearings. If lubrication doesn't stop the squealing noise, consider calling a furnace contractor to assess the problem.
A low humming noise
If you have an older-model furnace that's making a low-pitched humming noise when the blower motor is turned off, it's most likely a problem with the pilot light needing to be readjusted. A humming sound that you hear when the blower is on could be from a dirty gas burner. Dust and dirt can build up when the furnace doesn't receive regular maintenance, leading to a low-pitched rumbling sound.
Metal scraping sound
If you hear a sound like metal scraping on metal, the blower wheel probably needs to be repaired. It may simply have come loose and needs to be tightened back up. If the blower wheel is damaged, it can be replaced by a furnace contractor. The blower's motor mount can also be replaced if it breaks and is the source of the metal scraping sound. An out-of-balance blower wheel can create reoccurring problems with the motor mount, so it's important to check that it's properly balanced. Installation issues or even heavy dirt buildup can cause a blower wheel to get out of balance.
Noise from metal ductwork
Does the metal ductwork in your home make clanging or popping sounds when your furnace turns on? This tends to happen when hot air suddenly enters a cold duct, causing the metal to expand. Typically, it would make some mild creaking, but if the duct hits against wood, it can make a loud popping sound. Covering the ducts with fiberglass insulation or inserting sound-absorbing duct liners can cut down the noise, but can be expensive. Make sure nothing is blocking the system to cause pressure problems. A furnace contractor can check to make sure your furnace and ducts are working as they should.