What was found
WHILE CRUISING LEVEL AT FL450, APPROX 40NM SW OF KBNA, ENROUTE FROM KXNA TO KCAE WITH ONE, CREW NOTICED A HISSING NOISE COMING FROM THE MAIN DOOR AREA. VERY SHORTLY AFTER, CREW RECEIVED A MASTER CAUTION LIGHT CONCURRENTLY WITH A DOOR SEAL LIGHT. CREW DONNED O2 MASKS, REQUESTED LOWER ALTITUDE FROM ATC WHILE INITIATING AN EMERGENCY DESCENT. WITHIN A MINUTE, MASTER WARNING LIGHT ILLUMINATED CONCURRENTLY WITH A CABIN ALTITUDE LIGHT. CABIN ALTITUDE REACHED APPROX 15,000 FT, THAT ACTIVATED THE CABIN ALTITUDE MASKS. PAX DONNED O2 MASK AS ADVISED. ATC DECLARED AN EMERGENCY. NOTIFIED ATC OF ITS INTENTION TO DIVERT TO KBNA. ATC CLEARED THE ACFT TO 10,000 FT, DIRECT KBNA WHERE A NORMAL LANDING OCCURRED. THROUGH INVESTIGATION, FOUND MOISTURE IN SERVICE AIR LINES LEADING TO THE DOOR SEAL SYS. RECOMMENDED TO PERFORM SB 525A-36-04, WHICH INSTALLS A MOISTURE TRAP IN THE LINES. ACFT WILL BE SCHEDULED TO GET THE SB COMPLIED WITH. IN INTERIM, WILL HAVE CREWS COMPLY WITH MC525A-003.