The most common reasons for a furnace making a humming noise are dirt buildup, failing motors or gas leaks, according to Air Mechanical Inc. If a furnace is experiencing one of these problems, it needs repairs as soon as possible.
Air Mechanical Inc explains that dirt buildup can occur if there is a lot of outside contamination entering the furnace, which is generally due to a problem with the air filter. A thorough cleaning is needed, and the filter needs to be changed. Excessive dirt places extra wear on furnace components, leading to damage. If the humming noise is accompanied by a squeal, it means that the motor's bearings require lubrication or possible replacement.
Sometimes, the motor can get overworked and begin to burn out when it's not properly maintained. If this happens, the homeowner needs to call a professional to replace the motor, as a failing motor can stop working at any time.
If the furnace is powered by natural gas, and it begins to rattle from the inside, it could mean that there is a carbon monoxide leak due to cracks in the heat exchanger. In this case, an HVAC technician is needed immediately, as carbon monoxide is a toxic gas.