What was found
LT ENGINE VIBRATION IN FLIGHT. REMOVED LT ENGINE LOWER COWLING AND INSPECTED. REMOVED STARTER/GENERATOR (PN 9912125-3; SN 96107) FOUND BEARINGS WERE WORN. INSTALLED O/H CUSTOMER SUPPLIED PART: STARTER/GENERATOR (PN 9912125; SN 1746) IAW MM, CH 80-11-01. OPS CHECK OF STARTER/GENERATOR WAS SATISFACTORY. SINCE TIME ON THE PART, AND THE DUTY HISTORY IS UNKNOWN IT IS HARD TO STATE A PROBABLE CAUSE. FREQUENT REMINDERS TO AIR CREWS TO UTILIZE APU/GPU FOR GROUND STARTING ENGINES WHENEVER POSSIBLE. MAKE SURE THERE VOLTAGE IS AT 28.5 VDC TO MINIMIZE THE OPERATIONAL STRESS TO THE STARTER/GENERATOR UNDER LOAD WHEN STARTING ENGINES.