What was found
PILOT REPORTED THAT THERE WAS A VIBRATION IN THE ACFT. INVESTIGATION FOUND THE RIVETS WORKING AT TAILBOOM TO TAIL ROTOR GEARBOX CASTING MOUNT. FURTHER INVESTIGATION FOUND A CRACK IN THE TAIL BOOM AT THE 11:00 POSITION, APPROX 1.5 INCHES LONG AT THE ROW OF RIVETS ATTACHING THE TAIL ROTOR GEARBOX CASTING TO THE TAILBOOM. AN EVENT 4 INSPECTION HAD BEEN COMPLIED WITH 64 HOURS PRIOR WITH NO DEFECTS NOTED. THE LAST BALANCE AT EVENT 1 INSPECTION 163.6 HRS PRIOR WITH NO DEFECTS NOTED. AN EVENT 5 INSPECTION HAD BEEN ACCOMPLISHED 17.7 HOURS PRIOR AND NO DEFECTS NOTED. THE TAIL ROTOR WAS REMOVED AND A STATIC BALANCED PERFORMED. THE STATIC BALANCE WAS FOUND TO BE OUT OF TOLERANCE. UPON FURTHER INVESTIGATION THE TAIL ROTOR TRUNNION WAS FOUND TO HAVE SHIFTED . 065.