Are any of the accessory pulleys frozen? You can take the belt off and spin the pulleys. Next Id check belt tension. Can you pin-point the noise? Sometimes you can use a long shank screwdriver, place the tip on accessory housing, place the other end of metal housing on that bone behind your ear, helps to magnify the sound. Just don't get the tool or yourself--cloths or anything caught in belt--pulleys--fans or anything. Safety is your responsibility.