What was found
WHILE APPROACHING, THE CREW EXPERIENCED A NOTICEABLE VIBRATION THAT WAS ISOLATED TO THE RT STARTER/GENERATOR. THE CREW DESCENDED TO A LOWER ALTITUDE, DISCONTINUED USE OF THE RT STARTER/GENERATOR, AND UTILIZED THE APU GENERATOR FOR THE REMAINDER OF THE FLIGHT. CENTER CHOSE TO DECLARE AN EMERGENCY IN ORDER TO BYPASS A 30+ MINUTE HOLD. MAINTENANCE INVESTIGATION REVEALED WORN BEARINGS WITHIN THE RT STARTER/GENERATOR WHICH HAD APPROXIMATELY 721 HOURS SINCE O/H. THE RT STARTER/GENERATOR WAS REMOVED AND REPLACED WITH SERVICEABLE UNIT. ALL MAINTENANCE AND A SATISFACTORY OPERATIONAL CHECK WERE PERFORMED IAW MFG MM. (K)