If it's been over 3 or 4 years since their last inspection, the V-belts or mono belt need to be checked and/or replaced. Inspect for cracks and separation of the belt plies. Also check for fraying and glazing, which makes the look glossy or shiny. Be sure to look at both sides of the belts and also see that the belts are tensioned properly. A note on belt sizes: the belt size is stated in millimeters, i.e. a 21.36 x 2390 belt is 21.36MM wide and 2,390MM long. Our part numbers use an abbreviated version of the size. Our 10 x 980 belt equals a 9.5MM x 980MM belt; our 21 x 2390 belt equals 21.36MM x 2,390MM belt, etc.