B737 CREW RPTS PRESSURIZATION PROBS SHORTLY AFTER THE START OF VIBRATIONS IN THE MAIN CABIN. CABIN ALT WARNING HORN SOUNDED. EMER DECLARED FOLLOWED BY AN EMER DSCNT.

Narrative

ACFT WAS OTS ALL NIGHT DUE TO A REPEAT WRITE-UP REGARDING A VIBRATION IN THE MAIN CABIN. THE PROB WAS THOUGHT TO BE ASSOCIATED WITH A CARGO DOOR SEAL THAT MAY HAVE BEEN AFFECTING THE PRESSURIZATION SYS. MAINT WAS STILL WORKING ON THE ACFT AT DEP TIME WHICH INCLUDED A PRESSURIZATION TEST ON THE GND. THE ACFT WAS SIGNED OFF BY MAINT AS AIRWORTHY. DUE TO THE RECURRING PROB; THE CAPT CONVERSED WITH BOTH MAINT AND OPS BEFORE ACCEPTING THE ACFT AND THEY ALL AGREED THAT THE MAINT PROB WAS IDENTED AND RECTIFIED. I WAS THE MONITORING PLT FOR THE FLT AND DUE TO THE PRESSURIZATION ISSUE; I WAS WATCHING THE PRESSURIZATION SYS MORE DILIGENTLY. THE INITIAL CLBOUT WAS NORMAL AND THE PRESSURIZATION WAS WORKING ON ITS NORMAL SCHEDULE. CLBING THROUGH APPROX FL180; A NON-REVENUE PLT SEATED IN THE MAIN CABIN CALLED THE FLT DECK ON THE INTERPHONE TO ADVISE US THAT A VIBRATION HAD STARTED IN THE FORWARD PORTION OF THE MAIN CABIN. WE OBSERVED THAT THE PRESSURIZATION SYS WAS STILL WORKING ON SCHEDULE SO WE CONTINUED OUR CLB. CLBING THROUGH FL280; THE CABIN RATE OF CLB STARTED TO INCREASE FROM 500 FPM TO 1000 FPM; BUT THE CABIN ALT REMAINED ON SCHEDULE. CLBING THROUGH FL300; THE DIFFERENTIAL PRESSURE STARTED TO SLOWLY DECREASE; AND THE CABIN ALT STARTED TO INCREASE FASTER THAN NORMAL. THE CABIN RATE OF CLB ALSO INCREASED TO MATCH THE ACFT'S ACTUAL RATE OF CLB. WE STOPPED OUR CLB BEFORE REACHING CRUISE ALT AND ASKED ATC FOR A LOWER ALT. WE DECIDED TO DSND DOWN TO AN ALT WHERE THE PRESSURIZATION WAS PREVIOUSLY WORKING NORMALLY AND WE WOULD BE ABLE TO RUN THE ABNORMAL PRESSURIZATION CHKLIST IN THE QRH. AS WE BEGAN OUR DSCNT; THE CABIN ALT STARTED TO INCREASE AT A VERY RAPID RATE AND CONTINUED PAST 10000 FT AT WHICH TIME THE CABIN ALT WARNING HORN SOUNDED. WE THEN COMPLETED ALL MEMORY ITEMS AND EMER CHKLISTS; AND ASKED ATC TO CONTINUE OUR DSCNT DOWN TO 10000 FT MSL. WE ALSO DECLARED AN EMER WITH ATC AND REQUESTED A RETURN TO OUR DEP ARPT. WE LANDED SAFELY WITHOUT ANY FURTHER INCIDENT. THIS MAINT PROB WAS MUCH BIGGER THAN ORIGINALLY THOUGHT. A MORE THOROUGH PRESSURIZATION SYS CHK SHOULD PROBABLY HAVE BEEN COMPLETED OR EVEN A FUNCTIONAL CHK FLT. THIS PLANE PROBABLY SHOULDN'T HAVE BEEN RETURNED TO REVENUE SVC. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: 718924: THE RPTR STATES THAT THE ACFT WAS FINALLY REPAIRED WHEN THE CARGO DOOR SEAL WAS REPLACED. MOST SEALS PERFORM BETTER AS PRESSURE DIFFERENTIAL INCREASES. THIS SEAL WAS DAMAGED IN SUCH A WAY THAT SEAL EFFECTIVENESS DECREASED WITH INCREASING PRESSURE.