As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING FLIGHT, THE PILOTS WERE COMPLAINING OF A VIBRATION IN THE MAIN ROTOR. THE MAIN ROTOR TRACK AND BALANCE WAS CHECKED AND NECESSARY ADJUSTMENTS WERE MADE TO BRING IT WITHIN ACCEPTABLE LIMITS. THE VIBRATION REMAINED DURING FLIGHT. AFTER FURTHER INSPECTION OF THE MAIN ROTOR CONTROLS IT WAS DISCOVERED THAT THE BOLT AND BUSHINGS THAT ATTACHED THE UPPER GUIDANCE UNIT TO THE AIRFRAME HAD BEEN WORN BEYOND ACCEPTABLE LIMITS.