What was found
(CAN) IN CRUISE FLIGHT, JUST PRIOR TO DESCENT FOR LANDING, THE CREW NOTICED A VIBRATION FROM REAR OF AC. THE AIRCRAFT LANDED UNEVENTFULLY. POST FLIGHT INSPECTION FOUND EXCESSIVE AMOUNTS OF BLACK STAIN (CARBON) ON BOTTOM OF LT ENGINE COWLING AFT OF STARTER/GENERATOR COOLING VENT. THE LT STARTER/GENERATOR WAS REPLACED AND SUBSEQUENT ENGINE RUNS FOUND THIS CURED THE VIBRATION. THIS IS THE THIRD STARTER/GENERATOR FAILURE ON THIS AIRCRAFT AND THIS UNIT WAS TO THE LATEST MODIFICATION STATUS. WE HAVE NOT HAD A STARTER/GENERATOR ON THIS AIRCRAFT MAKE IT TO ITS FULL RATED LIFE OF 1200 HOURS.