What was found
RAMP CREW VERIFIED THE CARGO DOOR WAS SHUT, THOUGH IT INDICATED OPEN. CREW REPORTED ON TAKE OFF, A LOUD VIBRATION OCCURRED AND ACFT DEPRESSURIZED. MAX ALTITUDE 9000 AND MAX SPEED 250 KIAS. DECLARED AN EMERGENCY DUE TO THE ACFT WAS NOT PRESSURIZING AND RETURNED TO DEPARTURE. THE ACFT LANDED WITHOUT INCIDENT. ACFT REMOVED FROM SERVICE. ON INSPECTION FOUND THE FWD CARGO DOOR FULLY OPEN. PERFORMED VISUAL INSPECTION OF THE FWD CARGO DOOR IAW AMM 52-35-00. NO DEFECTS NOTED. PERFORMED OPS CHECK OF FWD CARGO DOOR IAW AMM 52-35-00. OP'S CHECKED GOOD. PERFORMED CABIN PRESSURE CONTROL SYS IAW AMM 21-31-00. OP'S CHECKED GOOD. ACFT RETURNED TO SERVICE.