What was found
(CAN) DURING CRUISE THE PILOT NOTICED THE AIRCRAFT CABIN ALTITUDE WAS CLIMBING SO HE INITIATED AN EMERGENCY DESCENT. HE THEN HEARD A LOUD POP/BANG THAT SOUNDED LIKE IT CAME FROM THE AFT OF THE AIRCRAFT. NOT TO LONG AFTER THAT ALL THE OXYGEN MASKS DEPLOYED IN THE CABIN. THE AIRCRAFT ALSO HAD A MEL FOR THE RIGHT PACK PRIOR TO THE FLIGHT THE CAPITAN DID NOT DECLARE AN EMERGENCY AND THE PLANE LANDED WITHOUT INCIDENT. MX FOUND A SUPPLY DUCT COMING FROM THE LEFT PACK GOING BEHIND THE APU INTO THE AFT PRESSURE BULKHEAD BLOWN. THEY ARE ORDERING PARTS AND WILL REPAIR ACCORDINGLY 17- O2 GENERATORS WERE ACTIVATED.