PAX BOARDED ON AN ACFT THAT WAS ONLY AUTH TO BE FLOWN AS A FERRY FLT.

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ENRTE TO NASSAU; A LOUD WHISTLING NOISE SEEMED TO COME FROM AROUND THE COPLT'S WINDSHIELD AND THEN THE COPLT'S SIDE WINDSHIELD WIPER ACTIVATED AND SWIPED 3 OR 4 TIMES AT 320 KTS ON DSCNT. IT STOWED AND THEN SWIPED AGAIN AT 220 KTS. AFTER LNDG IN NASSAU; WHILE WRITING UP THE WHISTLING AND THE WIPER; IT ACTIVATED AGAIN. I WENT INTO OPS WITH THE LOGBOOK TO CALL MAINT. MECHS HAD BEEN ADVISED ON THE GND IN NASSAU AND MET THE PLANE. WHILE LCL MECH WAS ON THE PHONE WITH COMPANY MAINT; I LEFT TO GO BACK TO THE PLANE AND CHK THE MEL CONCERNING THE WIPERS. AT THIS TIME I HAD BEEN LED TO BELIEVE A PRESSURIZATION CHK WOULD HAVE TO BE DONE ON THE ACFT AT A REMOTE LOCATION WITHOUT PAX ON BOARD. WHEN I GOT TO THE ACFT; PAX HAD BEEN BOARDED. I TRIED TO CALL OPS TO HAVE THEM DEPLANED; BUT GOT NO ANSWER. A FEW MINS WENT BY WHEN THE LCL MECH WALKED UP TO THE COCKPIT; HANDED ME THE LOGBOOK AND SAID I WAS GOOD TO GO. HAVING JUST DETERMINED THAT THE WIPERS WERE DEFERRABLE; AND THE WHISTLING NOISE BEING ATTRIBUTED TO A COMS PANEL DOOR BEING LEFT OPEN IN ZZZ AT THE ORIGINATION OF THE FLT; BOTH WRITE-UPS LOOKED LIKE THEY WERE CLRED. I READ THE LOGBOOK; BUT DIDN'T NOTICE THE WIPER SYS WAS NOT DEFERRED; BUT OK'ED FOR A FERRY FLT. NO FERRY DISPATCH RELEASE WAS BROUGHT OUT. WE TOOK OFF; AND ABOUT 15 MINS INTO THE FLT; GOT A CALL FROM COMPANY DISPATCH ON ATL RADIO TELLING US TO RETURN TO NAS. WE DID. DEPLANED PAX; GOT PROPER FERRY DISPATCH RELEASE; WHICH WAS STILL ON THE PRINTER IN OPS; AND FERRIED TO MAINT CARRY-OVER COMPANY. BETTER COM BTWN THE CREW AND OPS IN NASSAU WOULD HAVE HELPED HERE. HAD THE PAX NOT BEEN BOARDED; I WOULD HAVE THOUGHT MORE ALONG THE LINES OF A FERRY FLT.