What was found
DURING CLIMB THRU FL280 AT MACH 79 STRONG AIRFRAME VIBRATION IN VERTICAL AXIS, UP & DOWN MOTION, AT APPROX RATE OF 2 - 3 CYCLES PER SECOND, HAD TO SLOW BELOW 280 KNOTS MACH 76 TO STOP MOTION. REPLACED SEALS ON HYD ACCESS PANELS & AFT CARGO DOOR SEALS. REPLACED LT BELLY HYD RES PANELS ACCESS LATCH & BOTH NLG AFT FIXED DOOR GIMBAL & ROTATION PIN.