What was found
KMIA, TRIP NR 00447, UNABLE TO PRESSURIZE THE AIRPLANE. A VERY STRONG NOISE WAS COMING OUT OF THE AFT CARGO DOOR SEAL. AIRCRAFT RETURNED TO KMIA. REPAIRED MAIN CARGO DOOR SEAL LOWER AFT CORNER. CHECKED OK IAW MM 52-2-1, PERFORMED PRESSURIZATION CHECK, OK IAW MM 21-31-0.