A B767-300 RETURNED TO THE DEP STATION TWICE WITH AIRFRAME VIBRATION. ACFT HAD PREVIOUS VIBRATION RPTS NOT CORRECTED.

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AFTER TKOF FROM ZZZ ON CLBOUT; A DEFINITE VIBRATION WAS FELT IN BUSINESS CLASS AND FIRST CLASS. THERE WAS NOISE AND THE CABIN TEMP WAS WARM. THE CAPT WAS NOTIFIED. THE VIBRATIONS HAD WORSENED. THE CAPT ASKED US OUR OPINION BUT DID NOT LEAVE COCKPIT. WE CIRCLED APPROX 1 HR AND WENT BACK TO ZZZ (NO FUEL DUMPED). WE ARRIVED IN ZZZ; MECHS SAID CARGO DOOR WAS NOT SEALED TIGHT. WE LANDED HVY. THE MECHS FIXED PROB AND SIGNED OFF. WE TOOK OFF AGAIN AND EXACTLY THE SAME PROB OCCURRED; ONLY WORSE. WE CIRCLED AGAIN APPROX 1 HR AND LANDED HVY BACK IN ZZZ. WE CHANGED ACFT AND FLEW TO JFK DEPARTING AT XA55 (APPROX). THIS ITEM HAD BEEN WRITTEN UP IN THE LOGBOOK BEFORE WE EVEN TOOK OFF THE FIRST TIME BUT SIGNED OFF. SUPPLEMENTAL INFO FROM ACN 520796: FLT ATTENDANTS RPTED VIBRATION AT MID CABIN. COULD ALSO DETECT THE VIBRATION ON THE FLT DECK. ADDITIONALLY WE WERE UNABLE TO MAINTAIN THE PROPER TEMP IN THE ACFT. ACFT HAD CDL REMOVING RAM AIR DOOR OFF THE L PACK. DID NOT DECLARE AN EMER WITH ATC. AFTER LNDG MAINT DETERMINED THAT THE L PACK EXHAUST DOOR HAD NOT BEEN PROPERLY LOCKED OPEN AS PER THE CDL. EXHAUST DOOR WAS LOCKED OPEN AND OVERWT INSPECTION ACCOMPLISHED. AFTER SECOND DEP; SAME VIBRATION PROB. RETURNED TO ZZZ AND DID SECOND OVERWT LNDG. DID NOT DECLARE AN EMER WITH ATC. UPON REVIEW OF FLT MANUAL DETERMINED POLICY IS OVERWT LNDGS ARE AN EMER AND I SHOULD HAVE DECLARED AN EMER WITH ATC TO COMPLY WITH MANUAL; EVEN THOUGH I DID NOT REQUIRE ANY SPECIAL HANDLING. DISPATCH INFORMED ATC OF OVERWT STATUS ON FIRST LNDG AND EMER EQUIP WAS SENT OUT. ON SECOND LNDG NO EQUIP WAS REQUESTED. PERFORMANCE LIMITS OF ACFT WERE NOT A FACTOR DUE TO WT AND RWY LENGTH.