As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AT 12000 THE AIRCRAFT STARTED TO PRODUCE HEAVY UNUSUAL VIBRATION IN THE HULL THE AND A VERY LOUD VIBRATING NOISE. WE DECLARED EMERGENCY AND RETURNED TO FIELD. NO MASTER WARNING AND NO MASTER CAUTIONS MESSAGES AND ALL ENG VIBRATIONS WERE WITHIN NORMAL RANGE. (FLT 5087) 10/23/2018 WORK IN PROGRESS.