What was found
AIRFRAME VIBRATION FELT ABOVE 250 KTS. INCREASES WITH AIRSPEED. NO VIBRATION IN CONTROLS OR RUDDER PEDALS. ENGINE VIBRATION 0.3/0. 2. FEELS LIKE A PITCHING MOTION (BOUNCE). FA REPORT MOTION EQUAL THROUGHOUT CABIN. DURING WALKAROUND, FOUND THE LT MAIN GEAR STRUT DOOR (CENTER DOOR) AFT BOTTOM HINGE BOLT BACKED OUT OF RETAINING CLIP. VERIFIED HARDWARE POSITIONS. TORQUED BOLT IAW AMM 32 -13-11-420-803. DOOR SECURE. NO DOOR ADJUSTMENTS DISTURBED OR REQUIRED. NO OTHER OBVIOUS DEFECTS NOTED PERTAINING TO BUFFETING.