What was found
TER INFLIGHT BCN-JFK. CREW REPORTED APPROXIMATELY 2 HOURS INTO FLIGHT, LOUD BANG HEARD AND AUXILIARY POWER UNIT AUTO SHUTDOWN WITH SUBSEQUENT LOAD RUMBLE IN AFT OF AIRCRAFT. EMERGENCY DECLARED FLIGHT DIVERTED TER LANDED WITHOUT INCIDENT. FOUND APU INLET DOOR MISSING ON ARRIVAL. FLIGHT FERRIED TO ORD FOR FURTHER MAINTENANCE. REPLACED AUXILIARY POWER UNIT AND APU INLET DOOR AT ORD. SYSTEM GROUND CHECK NORMAL OPERATION .