What was found
ON CLIMBOUT, HEARD A VERY LOUD LOW FREQUENCY VIBRATION. OCCURRED 2 TIMES AND SUBSIDED BOTH TIMES WITHOUT ANY CHANGES BY CREW TO CONFIGURATION. RETURNED TO TUC FOR PRECAUTIONARY LANDING. INSPECTED ENTIRE AIRCRAFT EXTERNALLY. INSPECTED INTERIOR OF BELLY AND ALL AIRCRAFT DUCTS. FOUND LIGHTING HOLES IN BOTTOM OF RUDDER TORN AND MADE INTERIM REPAIR IAW MM. NO OTHER DISCREPANCIES NOTED.