What was found
WITH NR 1 PACK ON MEL, PASSING ABOUT 15,000 FT MSL, CREW NOTED A RUMBLE IN THE AC DUCT SYSTEM. CABIN CLIMB RATE WAS ABOUT 650-700 FPM. AT FL240, CREW NOTED A STRONG RUMBLE AND VIBRATION AND INCREASE IN CABIN CLIMB RATE. THE 10,000 FT CABIN ALTITUDE WARNING SOUNDED AND THE CREW DONNED OXYGEN MASKS AND BEGAN EMERGENCY DESCENT UNTIL CABIN ALTITUDE STABILIZED BELOW 10,000 FT. CONTINUED TO DTW AND LANDED UNEVENTFULLY. REPLACED NR 2 PACK DOWNSTREAM CHECK VALVE AND SLEEVE/SEALS BETWEEN THE NR 2 PACK HEAT EXCHANGER AND ACM. BOTH PACKS LEAK AND OPERATIONAL CHECK GOOD. CLEARED NR 1 PACK MEL. AIRCRAFT RETURNED TO SERVICE.