Whistling Sound From the Furnace

Filters

• Most homeowners use pleated air filters on the furnace because they’re efficient and available in custom sizes. However, pleated filters don’t always fit tightly into the return grille. When this happens, the air may return around the narrow spaces between the filter and the filter housing, instead of returning through the filter itself. This causes a whistling sound.

Filter Solutions

• Filters are less likely to cause a whistling sound if they fit properly. Purchase the right size filter for your furnace, even if you have to buy a custom size. Ensure a tighter fit by attaching a new air filter to the return grille or filter rack with foam tape. Dirt on the filter reduces the flow of air through the filter, causing the air to go around the filter instead of through it. Try changing your filter to eliminate a whistling sound.

Ductwork

• If the filter isn’t the cause of the whistling noise from your furnace, you’ll need to check the ductwork. Inspect the ductwork for loose parts or holes that might cause whistling when air leaks through them. If the whistling sound is coming from your supply registers, and you can hear the whistling throughout the house, then you might not have enough supply registers or those you have might be too small. A blockage in the ductwork can also cause a whistling noise. If this is the case, you won’t hear the whistling throughout the house, rather only in a few places. Installing a new furnace with a more powerful blower can also cause whistling sounds when the air blows through the ductwork at a higher speed.

Ductwork Solutions

• Use duct tape, sealant or screws to repair and close up leaks. Inspect the ductwork for blockages by looking for kinking or collapsed ducts. Use a volume damper inside the ductwork. A certified HVAC contractor can help you determine if your supply registers are inadequate and help you decide where the damper should go. Running the furnace fan on low speed may help if a powerful blower is responsible for the whistling sound.