As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
FLIGHT #5512. DURING CRUISE FLIGHT EXPERIENCED INTERMITTENT LOW FREQUENCY VIBRATION FROM COCKPIT FLOOR, AIRCRAFT RETURNED TO ORIGINATION CITY CVG AND LANDED SAFELY.PERFORMED STATIC GEAR SWING, INSPECTED GEAR DOORS AND PERFORMED FLIGHT CHECK, DISCREPANCY COULD NOT BE DUPLICATED.