What was found
VIBRATION BEGINS AT FL260 AT 290KTS, INCREASED INTENSITY TO 350KTS. FELT IN CONTROL COLUMN & NOT IN RUDDER PEDALS. VIBRATION FELT IN COCKPIT SEATS & IN CABIN SEATS. CHECKED ALL ELEVATOR BELLCRANKS, PUSHRODS, CONTROL COLUMS AND HINGE POINTS FOR CONDITION AND SECURITY. NO DEFECTS NOTED IN REFERENCE TO G-200 AMM 27-30-00. INSPECTED ALL ANTENNAS FOR SERCURITY. NO DEFECTS NOTED IN REFERENCE TO G200 AMM 23-00-00. PERFORMED ELEVATOR CONTROL CABLES TENSION - ADJUSTMENT. NO DEFECTS NOTED.