As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING CLIMB, CREW NOTICED A VIBRATION WHICH INCREASED IN INTENSITY COMING FROM THE LEFT SIDE OF THE AIRCRAFT. CREW ELECTED TO RETURN TO DEPARTURE AIRPORT. GROUND MAINTENANCE FOUND A PORTION OF NR 1 ENIGNE INBOARD AFT COWL HAD LIBERATED FROM AIRCRAFT.