What was found
(CAN) DEPARTURE AFTER MAINTENANCE, AT FL 22,000 CLIMBING, CREW HEARD A LOUD BANG, THEN THE CABIN ALTITUDE BEGAN TO RISE ( DELTA PRESSURE DROPPED). THE CREW STOPPED CLIMB AND DESCENDED BELOW 10,000 FEET AND ELECTED TO RETURN TO POINT OF DEPARTURE. MAINTENANCE CREW FOUND THE AIR CYCLING MACHINE (ACM) DUCT (P/N 6615040-5) GOING TO THE WATER SEPARATOR TO BE LOOSE FROM RUBBER MATING DUCT. THE GROUND CREW REATTACHED THE DUCT ASSEMBLY AND TIGHTENED ALL CLAMPS (P/N S1891-64). PRESSURE CHECK AND LEAK DUCT CHECKED.