LOOSE WING PANEL CAUSES VIBRATION.

Narrative

AFTER A NORMAL TKOF AND CLB; WE NOTICED AS WE LEVELED OFF IN CRUISE; AN ABNORMAL VIBRATION IN THE CTL YOKE. THE VIBRATION SEEMED TO GET WORSE AS AIRSPD WAS INCREASED; WE DECIDED TO CRUISE AT SLOWER THAN NORMAL SPD. WE HAD NO VISUAL CLUES OF ANY KIND TO WHAT WAS CAUSING THE VIBRATION. THE VIBRATION WAS NOT VIOLENT; JUST VERY CONSTANT -- MORE LIKE A TYPE OF FLUTTER. WE ASKED OUR CABIN ATTENDANT IF SHE COULD SEE OR FEEL ANYTHING UNUSUAL; TO WHICH SHE RESPONDED THAT EVERYTHING LOOKED AND FELT NORMAL. AFTER CONFERRING WITH OUR MAINT CTL OVER THE RADIO; WE WENT ON TO OUR DEST WHERE THEY SAID THEY WOULD HAVE A MECHANIC WAITING FOR US. (WE WERE ABOUT HALFWAY THERE WHEN WE NOTICED THE VIBRATION). AS PAX WAS DEPLANED AT OUR DEST; A COUPLE OF PAX MADE US AWARE THAT THEY HAD SEEN A LOOSE PANEL JUST IN FRONT OF THE L AILERON. WHEN MAINT ARRIVED; THEY FOUND THE PANEL; JUST IN FRONT OF THE AILERON TO BE MISSING SEVERAL SCREWS; THE PANEL COULD ACTUALLY BE LIFTED UP A LITTLE BIT; ENOUGH FOR IT TO BE CAUGHT IN THE AIRFLOW WHILE INFLT. ACCORDING TO WHAT THE PAX THAT SAW THE PANEL INFLT SAID; HE DID NOT NOTICE IT UNTIL ABOUT HALFWAY INTO THE FLT; WHICH WAS ABOUT THE SAME TIME THAT WE IN THE COCKPIT STARTED TO FEEL THE VIBRATION. THE AIRPLANE HAD BEEN PICKED UP AT OUR MAINT HANGAR; JUST PRIOR TO OUR FLT; IT WAS THOROUGHLY PREFLTED; WITH SPECIAL ATTN TO PANELS THAT CAN BE OPENED FOR MAINT FUNCTIONS; GEAR DOORS; ETC. ALL THE PAPERWORK THAT GOES IN THE MAINT CAN WAS PROPERLY ENCLOSED AND CURRENT. IT HAD A FRESH 50 HR/7 DAY MAINT PREFLT SIGNED OFF! THE REQUIRED SIGNATURE ON THE DAILY FLT LOG HAD ALSO BEEN SIGNED OFF; INDICATING THE ACFT WAS RELEASED FROM MAINT FOR RETURN TO SVC. THE PANEL IN FRONT OF THE AILERON THAT WAS MISSING THE SCREWS; IS LOCATED ON TOP OF THE WING; AND IS IMPOSSIBLE TO INSPECT DURING A NORMAL PREFLT!