CE208 ON LEVELOFF AT 4000 FT HEARD A LOUD BANG AND FELT AIRFRAME SHUDDER. DECLARED AN EMER AND DIVERTED. FOUND WING BODY INSPECTION COVER HANGING FROM WING ROOT.

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ON JUL/WED/05; I DEPARTED ZZZ AT APPROX XA45 ENRTE TO ZZZ2. WX AT ZZZ AT THE TIME OF DEP WAS 005 OVCST WITH 10 MI VISIBILITY. DEST WX WAS FEW AT 001 AND VISIBILITY WAS BETTER THAN 6 MI WITH FOG IN THE VICINITY. I RECEIVED AN IFR CLRNC AND DEPARTED RWY 11. SHORTLY AFTER JOINING V-XXX AND LEVELING OFF AT 4000 FT (APPROX 15 DME W OF ZZZ VORTAC); I HEARD A LOUD BANG; AND FELT THE ACFT SHUDDER. THE ACFT CONTINUED TO SHUDDER AND SHAKE AND THE BANGING CONTINUED SO I CONTACTED CTR AND REQUESTED PRIORITY HANDLING BACK INTO ZZZ. AT THIS POINT THE CTLR CLRED ME FOR THE ILS TO RWY 11 BACK INTO ZZZ AND ADVISED ME THAT HE HAD DECLARED AN EMER FOR ME. I CONFIRMED THAT I WAS DECLARING AN EMER. I LANDED IN ZZZ WITHOUT FURTHER INCIDENT. WHEN I EXITED THE ACFT; I NOTICED A PIECE OF STRAP (INSPECTION COVER) HANGING DOWN FROM THE WING ROOT. IT WAS DETERMINED THAT THE NOISE AND VIBRATION WAS CAUSED BY THIS PIECE OF FLAT ALUMINUM STRAP COMING LOOSE INFLT AND STRIKING THE UPPER WING SURFACE. PART OF THE ALUMINUM STRAP HAD DEPARTED THE ACFT INFLT; AND THE PART THAT REMAINED WAS STRIKING THE UPPER WING SURFACE WITH ENOUGH FORCE TO SLICE HOLES IN THE UPPER WING SURFACE ITSELF. MAINT PERSONNEL HAD RECENTLY COMPLETED A ROUTINE INSPECTION OF THIS ACFT. DURING MY PREFLT INSPECTION; I LOWERED THE FLAPS; WHICH WOULD HAVE MADE THE TRAILING EDGE OF THE STRAP VISIBLE TO ME. I DID NOT NOTICE ANY MISSING SCREWS; OR ANYTHING ELSE THAT WOULD HAVE CAUSED THE STRAP TO COME LOOSE. SINCE THE TRAILING EDGE OF THE STRAP DEPARTED THE ACFT; MAINT WAS UNABLE TO DETERMINE WHY IT CAME LOOSE. THE ACFT HAD BEEN FUELED AND THE PREFLT INSPECTION COMPLETED BY ANOTHER PLT BEFORE I ARRIVED IN ZZZ THAT MORNING. I DID NOT KNOW THIS UNTIL I HAD COMPLETED MY PREFLT INSPECTION. THE WX WAS NICE; AND I WAS IN NO HURRY TO DEPART; DUE TO PAX CHKING IN LATE. IF IN FACT THE 3 SCREWS THAT HOLD THE STRAP ATTACHED TO THE TRAILING EDGE OF THE WING WERE LOOSE; BOTH MYSELF AND THE PLT BEFORE ME MISSED IT.