As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PILOT COMPLAINED OF MEDIUM FREQUENCY VIBRATION. VIBRATION WAS ISOLATED TO THE TAIL ROTOR DRIVE SHAFT 5TH SEGMENT. THE MS21042L4 NUT THAT SECURES THE HANGER BEARING ON THE SHAFT HAD LOST TORQUE. THIS ALLOWED THE BEARING INNER RACE TO SPIN ON THE SHAFT, RUINING THE SHAFT. THE NUT HAD 5 INCH LBS. OF TORQUE. THERE HAD BEEN NO PREVIOUS MAINTENANCE IN THIS AREA EXCEPT INSPECTIONS. THE SHAFT, ADAPTER, AND THE HANGER BEARING HAD TO BE REPLACED.