As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
TROUBLESHOOTING VIBRATION PROBLEM DISCOVERED TRANSMISSION INPUT PINION SHAFT WAS LOOSE. REMOVED SHORT SHAFT AND RELATED ACCESSORIES AND FOUND INPUT PINION SHAFT HAD LOSS OF PRE-LOAD TORQUE. INPUT PINION SHAFT NUT PN DH5439-111-42 STILL APPEARED TIGHT, BUT ROTOR BRAKE DISC HAD AXIAL PLAY OF .055 INCH. REMOVED BEVEL REDUCTION GEARBOX PN 350A32-0300-04 AND REPLACED WITH SERVICABLE UNIT.