Possible Cause 
Recommended Action

Various problems             

The belts are losses and striking the belt guard          

Adjust the belts to the correct tension.

  Belts are wearing out too quickly and/or the belts are too tight.   Adjust the belts to the correct tension.  Also check the belts are a matched set, if they are not replace the complete set.
  The pulleys are worn.   Replace both the pulleys and belts.
  The belt may be the wrong cross-section for the pulley.   Check and replace the belts as necessary.
  The pulleys may be incorrectly aligned.   Check the pulleys and realign as necessary.
  The drive selection may be incorrect.   Check and change as necessary.
  The fan, motor or its base may not be securely fastened.   Check and secure as necessary.