Knocking Sound in the Furnace

Causes

• Although expanding and contracting metal is the main reason for knocking sounds, it isn’t the only reason. Sometimes a loose piece of metal will make a knocking or rattling sound as it’s blown about by a current of air. Another, more serious reason, why you might hear a knocking sound is if you have a problem with your heat exchanger.

Identifying

• If the furnace itself starts to rattle or knock about 30 seconds before the blower comes on, you may have a cracked heat exchanger. Some people compare this sound to the sound a water pipe makes when it’s vibrating. Cracked heat exchangers can leak carbon monoxide into your house, so this is a serious problem that should be checked as soon as possible. Knocking sounds emanating from metal ducts usually sound as if they’re coming from inside the wall, or from the exposed ducts in your basement.

Locating

• Locating which ducts are causing a problem isn’t easy. Try to pinpoint where it’s coming from by following it down the duct line. If the noise is coming from a part of the duct pipes located inside the wall, you will probably need to remove the drywall or plaster. Doing so will make the sound clearer and should allow you to see where the duct is rubbing against the wood framing.

Fixing

• Lubricants may fix the noise problem for awhile, but they will eventually wear off. A permanent solution comes by ensuring that the ducts don’t rub on the wood. One way to do that is by enlarging the holes the ducts run through, but you shouldn’t be too quick to start cutting away at the wood or you might compromise the structural integrity of your house. Your local building inspector can tell you what’s safe to cut. You also might be able to cushion the ducts with rubber pads. Installing an expansion joint collar will help prevent some expansion and contraction problems by separating the large heating duct from the metal parts of the furnace.