What was found
ON DEPARTURE/CLIMBOUT, PASSING FL 180, A LOUD HUMMING SOUND ACCOMPANIED BY VIBRATION BEGAN IN THE COCKPIT FLOOR. THERE WERE NO CONFIGURATION OR THROTTLE CHANGES WHEN THIS BEGAN. NOISE SOUNDS SIMILAR TO THE SOUND OF THE PUMP WHEN THE TOILET IS FLUSHING. HOWEVER, SOUND COMES FROM BELOW THE COCKPIT FLOORBOARD AREA. SOUND/VIBRATION LASTED APPROXIMATELY SEVEN MINUTES. NO HYDRAULIC CHANGES WERE NOTICED ON THE GAUGES. AMM 53-12-01-2, AMM 30-42-00-6, AMM 21-20-11-6, AMM 05-51-38-2. OPENED NOSE RADOME CHECK PER AMM 53-12-01-2,WEATHER RADAR SYSTEM - ADJUSTMENT/TEST PER AMM 34-43-00-5-6, NO ABNORMAL FOUND. ALSO PERFORMED WINDSHEILD WIPERS CHECK & PROPERLY SECURED PER AMM 30-42-00-6, & INSPECTED AIR CONDITION DUCTS IAW AMM 21-20-11-6, INSPECTED RAM AIR INLET DOOR IAW AMM 21-53-01-4 AND RAM AIR INLET DOOR ACTUATOR IAW AMM 21-53-03-4, FOUND NO ISSUES. AIRFRAME VIBRATION CONDITION CHECK IAW AMM 05-51-38-2, ALL IN GOOD CONDITION. PERFORMED AND INSPECTED NLG XFER CYLINDER FOR LEAKS AS WELL AS CYCLING NLG LEVER UP&DOWN WITH NO ABNORMALITIES IAW AMM 32-34-04-4. NO REFERENCE REQUIRED ABOVE MX ACTIONS DONE WITH CONSULTATION & CONCURRENCE WITH MOCC AND MDO. AIRCRAFT CALLED UP & OK FOR SERVICE.