A B727 IS MANDATED TO RETURN LAND WHEN THE CABIN ALT STARTS TO CLB WHEN ACFT IS NEAR FL240. FLT ATTENDANTS HAD RPTED A 'NOISE' PRIOR TO DEP BUT COCKPIT CREW ASSUMED IT WAS A LEAKY DOOR SEAL SINCE THE DOOR LIGHTS WERE OUT. THE ROLLERS ON THE DOOR WERE NOT ENGAGED IN THE LOCKING MECHANISM.

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FOLLOWING THE ANNOUNCEMENT TO PREPARE THE CABIN FOR DEP; THE AFT FLT ATTENDANT NOTIFIED ME OF A LOUD NOISE AT THE AFT L SVC DOOR. THE DOOR ANNUNCIATOR LIGHT ON MY PANEL WAS EXTINGUISHED INDICATING THE DOOR WAS LOCKED CLOSED. I INFORMED THE CAPT OF THE NOISE AND CHKED TO SEE IF THE ACFT WAS PRESSURIZING NORMALLY. THE ACFT HAD PRESSURIZED NORMALLY WITH THE FLT/GRD SWITCH ON THE PRESSURIZATION CTLR IN THE FLT POS (CABIN READING .125 PSID). FOLLOWING A DISCUSSION OF THE INFO AVAILABLE; THE CAPT AND I FELT THIS WAS A LEAKY DOOR SEAL WHICH WOULD SEAL UP ONCE THE ACFT DIFFERENTIAL INCREASED. I INFORMED THE FLT ATTENDANT THAT WE FELT THE NOISE WOULD SOON DIMINISH AS THE ACFT CONTINUED TO PRESSURIZE. FOLLOWING TKOF; THE ACFT CONTINUED TO PRESSURIZE NORMALLY WITH A RATE OF CLB OF 300-500 FPM AND A STEADILY INCREASING DIFFERENTIAL. DURING THE CLBOUT THE CARGO V CLOSED LIGHT ON THE FE PANEL ILLUMINATED INDICATING THE CARGO HEAT OUTFLOW HAD CLOSED. I INFORMED THE CAPT THAT THE LIGHT HAD ILLUMINATED AND ALSO ADVISED HIM THAT ALL OTHER INDICATIONS WERE NORMAL. CABIN ALT WAS AROUND 4000 FT AND DIFFERENTIAL WAS STILL INCREASING THROUGH 5 INCHES. DURING THE CLB; THE AFT FLT ATTENDANT AGAIN CALLED THE COCKPIT AND EXPLAINED THAT THE NOISE HAD GOTTEN WORSE AND THE PAX WERE BECOMING CONCERNED. FEELING THIS MUST BE A POOR SEAL; THE CAPT AND I AGREED THAT IT COULD BE SEALED USING PAPER TOWELS. I MADE THIS SUGGESTION TO THE FLT ATTENDANT AND CONTINUED TO MONITOR THE PRESSURIZATION CTLR. PASSING THROUGH FL220 TO FL230; THE DIFFERENTIAL STOPPED INCREASING AT 5.8 PSID (NORMAL IS 8.6 PSID) AND THE CABIN ALT CONTINUED TO CLB. THE LEAD FLT ATTENDANT ENTERED THE COCKPIT AND REQUESTED I GO AFT AND INVESTIGATE THE DOOR AS THE PAPER TOWELS HAD NOT WORKED AND WERE SUCKED FROM UNDER THE DOOR. PRIOR TO REQUESTING PERMISSION FROM THE CAPT TO GO AFT; I NOTICED THE CABIN RATE OF CLB INCREASE FROM 500 FPM TO 1000 FPM. I INFORMED THE CAPT THAT THE CABIN WAS BECOMING UNCTLABLE AND TOLD THE LEAD FLT ATTENDANTTHAT I WOULD NOT BE ABLE TO LEAVE MY STATION. AS THE CABIN ALT WAS PASSING THROUGH 9000 FT; THE CAPT INSTRUCTED THE FO TO COORDINATE A DSCNT AND RETURN TO DEP FIELD WITH ATC WHILE WE COMPLETED THE LOSS OF PRESSURIZATION PROCS. SOON AFTER BEGINNING THE DSCNT AND SWITCHING THE PRESSURIZATION CTLR TO MANUAL; THE CABIN ALT BEGAN TO DSND AT AROUND 300 FPM. I INFORMED THE CAPT AND FO THAT I HAD CTL OF THE CABIN. AFTER DUMPING FUEL TO REDUCE TO MAX LNDG WT; A NORMAL LNDG WAS COMPLETED WITHOUT FURTHER INCIDENT. MAINT INSPECTION OF THE DOOR REVEALED THE ROLLERS ON THE BOTTOM HAD FAILED TO ENGAGE ALLOWING THE BOTTOM HINGED PORTION OF THE DOOR TO REMAIN AJAR WHILE STILL GIVING A LOCKED INDICATION. UPON COMPLETION OF MAINT ACTION AND DOCUMENTATION; THE FLT WAS CONTINUED. MULTIPLE FACTORS CONTRIBUTED TO ALLOW THIS SIT TO OCCUR. ALL PLTS ASSUMED THIS NOISE WAS SIMPLY A LEAK OF THE DOOR SEAL WHICH IS A FAIRLY COMMON COMPLAINT. WE ALSO ASSUMED THIS 'LEAKY SEAL' WOULD GRADUALLY CORRECT ITSELF AS THE CABIN DIFFERENTIAL INCREASED AND THE DOOR SEATED ITSELF AGAINST THE SEAL. THE CLOSED AND LOCKED INDICATION AND INITIAL NORMAL PRESSURIZATION INDICATIONS BOLSTERED THIS IMPRESSION. THE AFT FLT ATTENDANT FAILED TO PROPERLY RPT THE STATUS OF THE DOOR. INITIALLY; THE ONLY INFO PROVIDED TO THE COCKPIT WAS A NOISE. AS IT TURNED OUT; THE NOISE WAS ACCOMPANIED BY A GAP AT THE BOTTOM OF THE DOOR SUFFICIENTLY LARGE ENOUGH TO ALLOW PAPER TOWELS TO EXIT THE ACFT. ADDITIONALLY; IT WASN'T UNTIL THE ACFT HAD RETURNED TO THE DEP AIRFIELD THAT THE FLT ATTENDANT INFORMED THE CAPT THAT THE NOISE WAS LOUD ENG NOISE. THE CAPT STATED HAD HE KNOWN THIS; HE NEVER WOULD HAVE CONTINUED THE FLT. FINALLY; IN MY OPINION; THE DESIRE TO COMPLETE AN ON-TIME TKOF IN LIEU OF RETURNING TO THE GATE TO CORRECT AN ASSUMED NUISANCE GRIPE ALLOWED US TO COMPLETE THE TKOF WITHOUT PROPERLY DIAGNOSING THE SIT AND TAKING CORRECTIVE ACTION. INSTEAD OF ACCEPTING LIMITED INFO FROM THE FLT ATTENDANT AND ASSUMING A 'BEST CASE' SCENARIOINREGARDS TO THE DOOR; THE PROPER COURSE OF ACTION WOULD HAVE BEEN TO SEND ME BACK TO INSPECT THE DOOR PRIOR TO TKOF. ADDITIONALLY; THE FLT ATTENDANTS SHOULD HAVE BEEN MORE ASSERTIVE IN ADEQUATELY EXPLAINING THE SIT TO THE COCKPIT CREW MEMBERS. ENHANCED CRM TECHNIQUES USED BY ALL CREW MEMBERS COULD HAVE PREVENTED THIS SIT.