The spindle pulley has probably drifted on the shaft, and the index pin is dragging on the large track of the pulley.
If it is a repetitive clicking or knocking, or the belt scraping from misalignment, the problem is likely a loose set screw on the upper (spindle) pulley. There is a hole in the pulley to access it. Loosen the screw, push the pulley to the right so that the belt is aligned with the motor pulley. Put the belt on the small lobe of the pulley and line the pulley up with the large lobe of the motor pulley. Then tighten the screw. The set screw is M6X12 with a 4mm Allen head.