could be lots of things.stator, old fuel hose, reed, ring... does it 'chirp' at idle? have you looked at the magnets? more than likely hte cdbox like like skoolz said. them things are funny. sometime they run fine till they get warm. sometimes its the opposite. sometimes they just screw with ur mind for fun! i wouldnt buy a used one though. or a sierra for that matter. diaphrams are also cheap. but, for 700 bucks its hard to go wrong. good luck.