What was found
TUL - ON CLIMB, LEFT ENGINE LOW PRESSURE ROTOR SPEED STARTED TO FLUCTUATE DRAMATICALLY ACCOMPANIED BY LOUD BANGING NOISES. BROUGHT THRUST LEVER BACK TO 1.3 TO 1.2 AND ENGINE STABILIZED. RETURNED TO TUL AND LANDED WITHOUT INCIDENT. NO REMARKABLE EXHAUST GAS TEMPERATURE OR HIGH PRESSURE ROTOR SPEED. FLIGHT DATA RECORDER DOWNLOADED. FOREIGN OBJECT DAMAGE SUSPECTED. REPLACED NUMBER ONE ENGINE ON 03 DEC 2006 AT TUL. SYSTEM GROUND CHECKED NORMAL OPERATION.