A B737-300 IN CLB AT FL330 HAD CABIN PRESSURE WARNING HORN SOUND; DSNDED TO FL250. HEARD LOUD NOISE THEN CABIN CLBED. DIVERTED.

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FO WAS PF; CLBING THROUGH FL330 FOR FL370. NORMAL CHKS THROUGH FL180 REVEALED NOTHING ABNORMAL. AT FL330 CABIN ALT WARNING HORN SOUNDED. DID QRC AND STARTED ATC CLRED DSCNT TO FL250. ALL SWITCHES IN R POS; SEEMED TO HAVE GOOD AIRFLOW; AND OUTFLOW VALVE SHOWED FULL CLOSED. PUT PACKS TO HIGH FLOW AS WE STARTED DOWN. REGAINED CTL OF CABIN AND HAD CABIN DOWN TO 7500 FT WHEN WE LEVELED AT FL250. FLT ATTENDANT RPTED LOUD WEIRD NOISE IN CABIN. WENT BACK TO INVESTIGATE. SOUNDED LIKE IT WAS COMING FROM NEAR FORWARD CARGO DOOR. WITH CTL OF CABIN; #2 PAX MASK DEPLOYMENT; CONSULTED WITH MAINT CTLR AND DISPATCH. DECIDED TO CONTINUE TO ZZZ AT FL250; FUEL AND WX OK. ABOUT 15-20 MINS LATER; HEARD A LOUD NOISE IN CABIN. ASSUMED AN IMPENDING IF NOT ALREADY OCCURRING DECOMPRESSION. GOT IMMEDIATE CLRNC TO 10000 FT. DID QRC AGAIN. MANAGED TO GET TO 10000 FT BEFORE MASKS DEPLOYED FOR RISING CABIN. TALKED TO MAINT CTLR AND DISPATCH AGAIN. AT 10000 FT HAD ABOUT 1 PSI CABIN DIFFERENTIAL AND 7500 FT CABIN. ASSUMED PART OF CARGO DOOR SEAL HAD BEEN DAMAGED AND WAS BLOWN OUT. CONTINUED TO ZZZ; ASKED FOR VERY GRADUAL DSCNT FOR LNDG. NEVER EXCEEDED 500 FPM. NORMAL LNDG; NEVER DECLARED EMER BECAUSE OF NO NEED. GAVE VERY GENERAL INFO TO PAX AND FROM FLT ATTENDANT'S COMMENTS; MOST SEEM TO BE COMPLETELY UNAWARE OF ANYTHING UNUSUAL. GAVE FLT ATTENDANT DETAILED INFO; BUT TOLD THEM TO DO THEIR NORMAL DUTIES; NO PREPARATION; CABIN ADVISORY; NOTHING. AFTER LNDG; CONTRACT MAINT CONFIRMED THAT THE LOWER PART OF THE FORWARD CABIN DOOR SEAL WAS MISSING. COORD MAINT WITH MAINT CTLR AND DEPARTED FOR HOTEL. CALLED INCIDENT INTO HOTLINE ENRTE.