What was found
FUSELAGE STRUCTURE: CLIMBING THROUGH FL250 CREW FELT LOW LEVEL VIBRATION AND HEARD VBRATION. THIS CONTINUED UNTIL REQUESTING A LOWER ALTITUDE DOWN TO 13000 FEET. BELOW 13000 FEET UNTIL LANDING, NO VIBRATION OR NOISE WAS NOTED. RETURNED TO DEPARTURE AIRPORT. ACCOMPLISHED 05-50-27/600 (UNSCHEDULED: AIRCRAFT NOISE AND VIBRATION - INSPECTION/CHECK) PER CONTACT CENTER IAW AMM O5-50-27-200-801-A AIRCRAFT NOISE & VIBRATION- INSPECTION FLAP SHROUDS, FUSELAGE DRAINS, FLAP SEALS, NLG WHEEL ASSY AND SHIMMY DAMPER ALL ARE SOUND AND SECURE. NO DEFECTS NOTED. PRESSURIZED AIRCRAFT IAW AMM 21-31-00-860-803-A MULTIPLE TIMES COULD NOT DUPUCATE CREW WRITE UP. GAINED ACCESS TO OUTFLOW VALVE TO CLEAN AND GVI IAW AMM 21-31-02-000-801-A. NO DEFECTS NOTED. INSPECTED FUSELAGE DRAIN VALVES IAW AMM 53-05-00-200-801-A. NO DEFECTS NOTED.