EMER DSCNT INITIATED BY THE FLC OF A BEECH JET 400A WHEN A LOSS OF CABIN PRESSURE OCCURS 90 NM E OF BIL; MT.

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SIC WAS PF. CLBING THROUGH FL190; HEARD SLIGHT 'WHISTLE' FROM CABIN AREA. 'WHISTLE' INTENSIFIED AS WE CONTINUED TO CLB. CABIN WAS PRESSURING NORMALLY. ALTHOUGH CLRED TO FL330; STOPPED CLB AT FL260. PIC WENT TO CABIN TO INVESTIGATE. HE DETERMINED THAT THE SOUND WAS COMING FROM THE TOP OF THE MAIN CABIN DOOR. UPON HIS RETURN TO THE COCKPIT; WE DECIDED TO REQUEST A DSCNT TO FL190 -- TO WHERE WE HAD FIRST HEARD THE 'WHISTLE.' ONCE CLRED TO FL190; THE PIC (PNF) ADJUSTED THE CABIN PRESSURE CONTROLLER FOR OUR NEWLY ASSIGNED ALT AND IMMEDIATELY THE CABIN BEGAN TO RAPIDLY LOSE PRESSURIZATION. THE CABIN VSI INDICATED A CABIN CLB IN EXCESS OF 6000 FPM. I INITIATED AN EMER DSCNT. CTR CLRED US TO DSND TO 10000 FT. PASSING 17000 FT MSL; IT APPEARED THAT THE CABIN WAS STABILIZING AT 11000 FT (APPROX 1.5 INCHES DIFFERENTIAL) AND WE LEVELED OFF AT 15000 FT MSL VICE CONTINUING DOWN TO 10000 FT. WE CONTINUED TO OUR DEST AT 15000 FT (11000 FT CABIN ALT). WE PLANNED FOR A MINIMUM RATE OF DSCNT PROFILE INTO OUR DEST. AS WE DSNDED THROUGH 11000 FT MSL (WHERE THE CABIN ALT WAS EQUAL TO THE ACTUAL ALT) THE CABIN RATE OF DSCNT WAS COMMENSURATE WITH THE ACFT'S ACTUAL DSCNT RATE VICE TRYING TO REGULATE THE DSCNT RATE WITH THE RATE CONTROLLER. UPON AN UNEVENTFUL LNDG; AN INSPECTION OF THE CABIN DOOR SEAL DID NOT REVEAL ANY 'SMOKING GUN' AS TO A CAUSE OF THE LOSS OF PRESSURIZATION. WE FLEW THE ACFT (WITHOUT PAX) TO A MAINT LOCATION. ON THIS 'FERRY' FLT; ALTHOUGH WE ONLY FLEW IT AT 16000 FT MSL; THE ACFT APPEARED TO PRESSURIZE NORMALLY. THE MAINT CREW COULD NOT DUPLICATE THE LOSS OF PRESSURIZATION; NOR COULD WE ON A MAINT TEST FLOWN 2 DAYS LATER UP TO FL370. NO CAUSE WAS FOUND FOR THIS INCIDENT.