What was found
CREW REPORTED AIRCRAFT DID NOT PRESSURIZE AND NO DELTA-P AFTER TAKEOFF. AT 6,000 FEET MSL, CABIN ALTITUDE WAS 6,000 FEET. FLIGHT ATTENDANTS NOTED UNUSUAL NOISE IN AFT AIRCRAFT THAT SOUNDED LIKE AN OPEN DOOR. CREW RETURNED TO FIELD AND LANDED WITHOUT INCIDENT. MAINTENANCE INSPECTED AFT CARGO DOOR SEAL AND PERFORMED OPERATIONAL CHECK ON AFT CARGO DOOR. MAINTENANCE PRESSURIZED AIRCRAFT WITH CREW. NO DEFECTS NOTED. AIRCRAFT RELEASED FOR SERVICE. ALL WORK ACCOMPLISHED BY MAINTENANCE USING AIRCRAFT MAINTENANCE MANUALS.