As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
FLT 1224 RDU-BOS, CREW DECLARED EMERGENCY AND AIRCRAFT DIVERTED TO IAD. AT FL310 A POP WAS HEARD AND CABIN ALTITUDE BEGAN TO RISE. AN EMERGENCY DESCENT WAS CONDUCTED. CABIN ALTITUDE ROSE TO 13800FT. WHILE CRUISING AT 10000FT A NOTABLE VIBRATION WAS FELT ON THE FLIGHT DECK. MAINTENANCE FOUND CONDITION AIR CHECK VALVE NR 2 VALVE FLAPPER BROKEN AND FREE FROM FLAPPER HINGES.