A B737-400 IN CLB AT FL310 RETURNED TO THE DEP ARPT DUE TO CABIN ATTENDANT RPTING LOUD BANGING NOISE FROM DOOR 1L. FOUND DOOR HANDLE OUT OF DETENT AND IN VERT POS.

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PASSING THROUGH FL180; FLT ATTENDANT RPTED A LOUD BANGING NOISE FROM FORWARD (L1) DOOR. NO AIR WAS ESCAPING THROUGH DOOR BUT SHE COULD HEAR LOUD WIND NOISE COMING FROM THE DOOR. WE SLOWED OUR CLB RATE AND REQUESTED LEVELOFF AT FL310 (FILED AT FL350). AS WE DID THIS WE CHKED OUR CABIN PRESSURE AND PRESSURIZATION SYS INDICATED NORMAL READINGS. AS A PRECAUTION; WE REQUESTED FL230. WE CALLED MAINT CTL AND DISPATCH; EXPLAINED THE SITUATION AND THEY THOUGHT IT WOULD BE BEST TO RETURN. WE RECEIVED THE AMENDED RELEASE AND A NEW CLRNC. FOR THE REMAINDER OF THE FLT WE HAD NO PRESSURIZATION ISSUES AT ALL; BUT AIR NOISE CONTINUED AS RPTED BY 'A' FLT ATTENDANT. ON TAXI TO THE GATE; WE HAD ASKED FOR MAINT TO MEET ACFT. AFTER WE SHUT DOWN AND DEPLANED; WE TALKED TO MAINT; AND THEY HAD SEEN THE HANDLE FOR THE L1 DOOR NOT STOWED PROPERLY AND WAS IN THE VERT POS AS WE TAXIED TO THE GATE. THEY SPECULATED THAT THE HANDLE WAS BEING LIFTED BY THE AIRFLOW AND THEN PULLED INTO AND OUT FROM THE DOOR; THUS MAKING THE BANGING NOISE HEARD AIRBORNE AND ALSO WAS IN THE AIR STREAM CAUSING THE LOUD AIR FLOW NOISE. NO PAX ISSUES WERE RPTED AND ACFT REMAINED PRESSURIZED THROUGHOUT.