When the weather turns cold, you want your heating system to work properly. Furnaces make many natural noises during operation, but a humming or buzzing sound is not usually normal. Failure to troubleshot and fix the noises could cause your furnace to quit completely. You should be able to find source of the humming or buzzing yourself by following these troubleshooting tips.
Inspect the Blower Motor
Humming noises can commonly be traced to the blower motor. Once the blower motor heats to the set temperature, it reduces fan speed, which creates a humming noise. Access the blower motor through the furnace door, and look for the motor in a squirrel cage, or consult your manual for parts location.
If the motor makes a significant hum or buzz as it slows down, or the motor hums without turning, this may be because of a fault capacitor. A motor that hums when the unit is off commonly indicates the pilot needs adjusting.
Turn the unit off if it has been running and let it cool, and then test the motor with a voltage meter. Compare the reading to the required voltage, which is commonly written on the plate. If the reading exceeds the required amount, it means the motor is working harder.
Tighten Loose Components
An older furnace or one that hasn't been examined in awhile may have loose components. Constant vibration cause the parts to loosen, so regular inspection is essential. Check the blower motor fan for loose parts, loose vent covers, and loose burner plates.
To temporally fix humming noises, slide a piece of cardboard or plastic between the components. If you can't locate the source, contact an HVAC service.
Check Transformer, Ductwork, and Plenum
The ductwork and plenum (the air distributor box secured to the HVAC supply outlet) are made from a thin sheet metal for pliability. Large plenums and long ductwork runs could hum from vibrations, and the seams or screws often come loose.
Check for loose screws and tighten them, or apply tape over the seams to decrease noise. Another option is to install tie straps between the ductwork and joists. Also, remove any obstructions around the ductwork that block air flow. You will also need to replace damaged plenums.
Also look for the transformer near the electrical components and check for loose bolts. The transformer could make a humming or buzzing sound as current passes through it.
Lubricate and Clean
Some humming or buzzing noises are the result of dirt or lack of motor lubrication. Remove debris from the blower motor using an old toothbrush and lubricate it. Check for dirty filters, remove them, and clean them in soapy water, and rinse. The filters should easily pull from the slot. Let the filters dry, and reinstall them.