As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
THE CREW REPORTED A ROUGH/IRREGULAR SOUND FROM THE ENGINE IN CRUISE. THE CREW RETURNED TO THE RAMP. IT IS NOT UNCOMMON FOR THIS SITUATION TO OCCUR NOT ONLY DURING INSPECTION GROUND RUNS, BUT ALSO DURING OPERATION BETWEEN INSPECTIONS. A COMMON SOURCE OF PROBLEMS WITH THIS FUEL CONTROL IS THE MIXTURE VALVE. THIS VALVE BECOMES SCORED AND THEN INACURATE IN ITS FUNCTION. THERE IS A PROCEDURE FOR LAPPING THIS VALVE SURFACE.