What was found
AT CRUISE SPEED AT 105 KTS, PILOT FELT SOMETHING IN CONTROLS AND AIRCRAFT STARTED SHAKING UP AND DOWN SEVERLY. PILOT IMMEDIATELY LOWERED COLLECTIVE AND REDUCED AIRSPEED. THE AMOUNT OF SHAKING SUBSIDED WITH THE CHANGE IN AIRSPEED. AIRCRAFT WAS FLYABLE WITH VERTICAL VIBRATION. PILOT RETURNED AND LANDED AND PERFORMED EMERGENCY SHUTDOWN. UPON POSTFLIGHT INSPECTION IT WAS DETERMINED THAT AN OUTSIDE TRIM TAB LOWER BONDING HAD PEELED AWAY FROM BLADE. NO DAMAGE ELSEWHERE. REPLACED MAIN ROTOR BLADE.