What was found
AIRCRAFT GROUNDED: HIGH FREQ VIBRATION FELT AFTER BEING AT CRUISE AT 39,000 FT FOR 5 MINS. SIMILAR TO A CELL PHONE VIBE. 0 HZ. COULD FEEL IN SEAT & FLOOR, AFT GALLEY COULD HEAR & FEEL IT. DESCENDED TO 35,000 FT. VIBE SLOWLY DECREASED IN INTENSITY AFTER BEING STABLE FOR 5 MINS, SEEMINGLY UNAFFECTED BY PX CHANGE & REMAINED AT ABOUT 80% OF ORIGINAL INTENSITY. SPEEDS VARIED BTWN 235-260 KTS. SEEMED UNAFFECTED BY ENGINE N1 OR AIRSPEED. AFTER ABOUT 15 MINS, DROPPED TO ABOUT 10% OF INTENSITY. AS A PRECAUTION, ARFF DISPATCHED. LANDED WITHOUT INCIDENT. MAINT FOUND RT WING MIDDLE FLAP TRACK FAIRING HAD EXCESSIVE PLAY & REPLACED BUSHINGS & HARDWARE IN RT WING MIDDLE FLAP TRAP FAIRING AS REQUIRED IAW AMM 27-51-18.