What was found
1) IN CRUISE AT FL370 A SUDDEN POP AND THE SOUND OF RUSHING AIR WAS HEARD. THE CABIN BEGAN A SLOW CLIMB OF 100 FPM. WE DESCENDED TO FL290 AND THE CABIN PRESSURE STABILIZED AND THE SOUND OF LEAKING AIR STOPPED.2) FOUND SECTION OF LINE FWD CARGO DOOR SEAL NOT SECURE. REINSTALLED SEAL IN RETAINER IAW 52-36-8. PERFORMED PRESSURIZATION CHECK IAW 21-30-0. OPS CHECKED GOOD.