What was found
(CAN) IN CRUISE AT 11,500 FEET, A/C DEVELOPED NOTICEABLE VIBRATION, AUTOPILOT DISCONNECTED AND A/C STARTED A ROLL TO THE RIGHT. CONTROL REGAINED AND AT 8,000 FEET, VIBRATION WAS VERY CRITICAL. COPILOT HAD BOTH FEET ON RUDDER PEDALS TO DAMPEN VIBRATION. RUDDER TRIM TAB TO ACTUATOR BOLT MISSING. REPLACED.