A central heating system with a gas furnace includes a fan to pull out the exhaust from the burning natural gas or oil used to heat the air. These fans must run whenever the furnace is lit, and they often cycle when the furnace is not blowing warm air because the pilot light is always burning. These fans can be direct drive to an electric motor or attached to a belt-and-drive wheel assembly if the fan is located far from the motor. Several problems can lead to a noisy, squeaky fan.
Constantly spinning fan blades accumulate a lot of dust and dirt. Mix this with any condensation or humidity in the air and it turns into a gummy black goo. When this happens the fan can become unbalanced in its mounts and wobble or vibrate. The vibrating fan can rattle surrounding metal ductwork and cause uneven airflow and occasional sounds as pieces of the buildup break off. Cleaning the blades is relatively easy. Just make sure the power is turned off and wipe them with a dry rag. Further cleaning with a degreaser may be needed because, after all, the fan is pulling carbon exhaust fumes. Get the blades clean and check for reduced noise levels.
Fans spin at high rates of speed and need to turn smoothly. Also, if there is a drive belt, it needs to be tight and firmly seated in the grooves of the drive wheels. If the fan spins and makes a squeaking noise, use some light sewing machine oil to lubricate the blade shaft. Do not lubricate the drive wheel or its belt groove. Make sure these are clean and free of any grease or dirt. A slipping belt can make an awful racket.
Finally, if the belt is loose, replace it by cutting off the old belt and putting a new belt on the motor and fan wheels. Place the belt around the smaller of the two wheels and then get as much around the large wheel as you can. Slowly turn the fan blade and the belt will stretch slightly and go around both wheels.
Clean the Fan Duct
Make sure there are no cobwebs, dust or other obstructions in front or behind the fan. Air moving around and through this material can cause whistling or rattles. What's more, this buildup can eventually work its way into the ductwork and maybe get blown into the building. No need adding more dirt than necessary; cleaning at the source leads to less cleaning later.