As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
C106-5 AND C106-6 JOURNALS WORE INTO AND THROUGH THE C648-1 BEARING MATERIAL MAKING METAL-TO-METAL CONTACT. HEAVY SHIP LOW FREQUENCY VIBRATION WAS NOTICED WHILE IN HOVER AND LOW COLLECTIVE INPUT. SUBMITTER STATED POSSIBLE CAUSE IS CORROSION ON JOURNALS, WORN OUT BEARING MATERIAL.