that sure sounds like it is a belt slipping. you may want to check and see if you have had a stick get up in the belt, oil get on the belt, etc. could also be one of teh deck spindles binding and causing this. check each blade for easy spinning movement, but NO side to side movement. also verify you don't have a clump of dried grass under there, that can slow a blade enough to chirp a couple times before it knocks it out.