What was found
CRUISING AT FL 330 CREW NOTICED THE CABIN ALTITUDE AT 9300' AND CLIMBING AT 450FPM. COM PROCEDURES DID NOT ARREST CABIN CLIMB AND CREW PERFORMED AN EMERGENCY DESCENT TO A LOWER ALTITUDE WHERE THEY WERE ABLE TO REGAIN CONTROL OF CABIN PRESSURIZATION. FLIGHT DIVERTED TO MKE AND LANDED WITHOUT FURTHER INCIDENT. MAINTENANCE FOUND A DUCT FROM THE LEFT ACM SEPARATED AND REINSTALLED IT. THE NR 1 CONTROLLER BITE SHOWED FAULT CODE 016, NR 2 HAD NO FAULTS. AIRCRAFT HAS RETURNED TO SERVICE.