If fault in the belt gets ruled out, then next comes checking the pulleys or bearings in them.
An easy way to check them is to remove the belt passing on to them, and rotate them freely. Faulty pulleys / bearings in them will either give noises, or abnormal play, or both.
Diagnose which pulley gives any such abnormal noises. Repair / replace it, and hopefully, the issue will get resolved. I.A.