What was found
AFTER DEPARTING, FELT VIBRATION THROUGH A/C. VISUALLY INSPECTED THROUGH L2 DOOR. O/B FLAP AILERON AND BALANCE TAB SEEN VIBRATING VIOLENTLY. FL 350-.81 .75 AND STILL VIBRATED, RECHECKED ON DESCENT THROUGH 15000, AND 250 KTS. SIGNIFICANTLY REDUCED VIBRATION BUT STILL VISUALLY SEE TAB SHAKING. FOUND LT O/B AFT FLAP ECCENTRIC LOOSE. TIGHTENED AND ADJUSTED O/B ECCENTRICS IAW MM 27-51-12. CYCLED FLAPS, NO EXCESSIVE PLAY NOTED AS PREVIOUS TO ADJUSTMENT.