What was found
AT FL210, HAD AN AIRFRAME VIBRATION NOTICED AT 300 KIAS AND INCREASED AS AIRSPEED INCREASED. INTENSITY INCREASED IN LT TURNS, CLIMBS, DESCENTS, AUTO-PILOT ON & OFF, YAW DAMPER ON & OFF, RUDDER BIAS ON & OFF. HAD NO EFFECT. THERE WAS NO ABNORMAL VIBRATION IN THE YOKE OR RUDDER PEDALS.