Problem: Furnace makes unfamiliar noises when operating.
Background: Sometimes homeowners think their forced-air furnaces need to be replaced because they make strange noises. But the cause may be a simple mechanical problem with the motor or fan within the blower system. Sometimes making simple adjustments and / or replacing a part will solve a strange noise problem.
What to do: Turn off the power to the furnace, remove access panels, and make a careful examination of the blower area. Check to make sure that the drive belt between the motor and the blower has proper tension, is in alignment, and is not cracked.
Also check the pulley on the motor, making sure that it is not loose on the shaft, has worn grooves, or is dirty. If loose, retighten any set screws. If worn or dirty, remove the belt, clean the pulley, and sand down its interior sides. Also check the pulley on the blower fan. Spin the blower to check for sound which may be caused by something inside. If the fan is rubbing against the housing, it needs to be adjusted and your best bet is to call a technician. Push and pull on blower pulley to check for end-play. If excessive, shaft collars need to be adjusted.
Special advice: A blower that needs oil may also make noises. Follow owner manual instructions to oil. Sometimes oil-less bearings wear out and need replacement. To see if the motor is making the noise, re move the belt and run the motor; the motor may simply need oiling, but motor bearings may still make noise.
If so, decide if you can live with the noise or if you want to replace the motor. A dirty furnace filter can also cause noise; clean the filter as recommended.
Helpful hint: If the burner of a gas furnace makes an unusual noise, the gas input amount may not be correct, or it may be getting too much primary air. In either case, contact a service technician. Also, contact a technician immediately if the walls or windows in your home sweat excessively. The furnace may not be getting enough ventilation or the flue pipe may be blocked.