just guessing without hearing or seeing it. One source of noise is the spring loaded idler pulley the belt runs around. If the shaft the pulley rides on doesn't have any lubrication and it runs dry, it may gall a bit causing the pulley to intermittenly bind and turn. When it binds, it changes the tension on the belt, which causes the idler assembly to pitch back and forth, causing a loud banging noise. To fix, take the pulley off and check the shaft for any galling. Take some fine emery cloth and get rid of any burrs and polish up the shaft where the pulley rides. Check the hole in the pulley for any burrs, sand lightly if there are until smooth. Put a very small amount of high temp grease on the shaft where the pulley rides. Just a thin film is all you want. Shaft should turn smoothly without any binding. Re-assemble and the noise should be gone.