To find out why your furnace is making loud noises, you must first identify the specific noise. Some of the most common furnace noise problems include squealing, grinding, humming and rattling. These problems are commonly caused by loose fan belts, shot bearings, loose components or loose panels, respectively.
First check the belt connecting the motor to the fan if your furnace is making squealing noises. The noise usually means that the belt has slipped out of position. Slip the belt back in position, or replace if worn. Grinding noises are usually caused by shot bearings. Shut off the furnace and immediately call a qualified furnace repair technician. Do not attempt to replace the bearings unless you are a qualified technician.
To get rid of a humming noise, locate the burner compartment of your furnace, and remove the cover. Identify the specific component making the humming noise, and tighten the mounting screws. Place a small piece of plastic or cardboard before remounting to eliminate the noise.
Check the cover panels if your furnace is making a rattling noise. The rattling noises are usually caused by loose-fitting panels. Check that the screws are tight, and tighten if necessary, using the correct screwdriver.