What was found
EXPERIENCED A LOW FREQUENCY CONTINUOUS & SOMETIMES PULSATING VIBRATION DURING CLIMB & DECENT. OCCURRED AT ABOUT FL 200 UNTIL CRUISE. STARTING ABOUT 280KT IAS & CONTINUING UNTIL ABOUT .82M. IN CRUISE, SEEMED TO DIMISH ABOVE .82M & REOCCURED ABOUT .78M SLOWING. SOUND SIGNIFICANTLY LOUDER IN REAR OF ACFT & SEEMED TO BE LOCATED BENEATH CABIN FLOOR. FOUND SCREWS LOOSE ON SKIN PANEL. RETORQUED LOOSE SCREWS. CLEANED OUTFLOW VALVES & MANOMETRIC BOX INLETS. PERFORMED VIBRATION SURVEY ON LT & RT ENGINES. PERFORMED BOROSCOPE INSPECTION OF NR 2 ENGINE FIRST STAGE COMPRESSOR. NO DEFECTS FOUND. BORESCOPE OF RT ENGINE & VIBRATION SURVEY PERFORMED & FOUND WITHIN LIMITS IAW MM 72-00-00.