A B757-200 IN CLB AT FL260 DIVERTED DUE TO UNUSUAL VIBRATION IN YOKE; RUDDER PEDALS AND FLOORBOARDS CAUSED BY LOOSE FASTENERS ON THE MID CABIN LAVATORY SVC PANEL DOOR.

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WE HAD DEPARTED ZZZ. ON CLBOUT; MY FO AND I NOTICED AN UNUSUAL VIBRATION EMANATING FROM THE YOKE; RUDDER PEDAL; AND FLOOR BOARDS. FLT ATTENDANT IN THE TAIL RPTED FEELING A RUMBLING TYPE VIBRATION AND NOISES COMING FROM THE FLOOR AND 4L DOOR. THEY ALSO SAID PAX COULD FEEL RUMBLING IN THEIR SEATS IN THE LAST ROWS OF COACH. I GOT A PATCH IN TO DISPATCH AND MAINT CTL; AND ATTEMPTED TO DIAGNOSE THE PROB AS PER TECHNICIAN'S INSTRUCTIONS; BUT COULD NOT PINPOINT THE PROB. TECHNICIAN SUSPECTED AN ACCESS DOOR OR GEAR DOOR OPEN OR POSSIBLE RUDDER FLUTTER. TECHNICIAN RECOMMENDED WE LAND AND I CONCURRED. MIA AHEAD WITH TSTMS ENRTE AND LOW CEILINGS AND VISIBILITY. ZZZ WAS CLR SO WE DECLARED AN EMER TO GET PRIORITY HANDLING AND RETURNED TO ZZZ. NORMAL LNDG WAS MADE AND PAX WERE ACCOMMODATED BY SVCS. ACFT WAS TAKEN OTS BY MAINT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE CAUSE OF THE AIRFRAME VIBRATION WERE LOOSE FASTENERS ON THE MID CABIN LAVATORY SVC PANEL DOOR. THE RPTR SAID THE DOOR WAS CLOSED; BUT THE LOOSE FASTENERS ALLOWED THE DOOR TO BULGE INTO THE AIRSTREAM CAUSING VIBRATIONS.