As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
APPROXIMATELY 1 HOUR INTO NORMAL CRUISE FLIGHT AT FL 400 THE MAIN CABIN DOOR SEAL FAILED, THE SECONDARY SEAL REMAINED IN PLACE. A SOUND LIKE AN AIR HORN WAS VERY LOUD AT THIS POINT. THE FLIGHT CREW DECLARED AN EMER & DONNED THEIR OXYGEN MASKS. THEY ASKED FOR AN IMMEDIATE LOWER ALTITUDE & DECIDED TO DIVERT TO THE NEAREST AIRPORT. THEY LANDED WITHOUT INCIDENT. R & R THE PRESS REGULATOR FOR THE DOOR SEAL. OPERATIONS & LEAK CHECKED GOOD.