What was found
ACFT DEPARTED. UPON REACHING FL 370 I, THE PILOT, REDUCED THROTTLE POWER FOR CRUISE FLIGHT. AT THIS MOMENT THERE WAS A LOUD BANG AND A JERK HEARD AND FELT FROM THE FLIGHT CREW. IMMEDIATELY FOLLOWING THIS, THE EVM INDICATED MAJOR VIBRATIONS COMING FROM THE LEFT ENGINE. IMMEDIATELY REDUCED POWER ON THE EFFECTED ENGINE AND REQUESTED A RETURN TO THE AIRPORT. AFTER REDUCING POWER THE EVM DROPPED FROM THE CAUTION BAND TO THE TOP END OF THE GREEN BAND. DECIDED NOT TO SHUT DOWN THE ENGINE. RETURNED TO DEPARTURE AND LANDED WITHOUT INCIDENT