What was found
FLIGHT LEVEL 290 AND SPEED 80 MACH THE AIRCRAFT VIBRATED MODERATELY. REDUCED SPEED AND RAISED SPEED BRAKES APPROX 5 DEGREES, THE VIBRATION STOPPED. LOWERED SPEEDBRAKE AND 5 MINUTES LATER THE VIBRATION RETURNED. THE ONBOARD MECHANIC NOTED THE OUTBOARD SECTION OF FLAP ON A/C RT SIDE WAS VIBRATING. PERFORMED OUTBOARD AFT FLAP ADJUSTMENT IAW MM 27-51-12.