As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AT APPROXIMATELY FL280, CREW REPORTED LOUD HORN-LIKE NOISE HEARD NEAR PASSENGER DOOR. CREW DIVERTED AND LANDED WITHOUT INCIDENT. MAINTENANCE CLEANED AND INSPECTED PASSENGER DOOR SEAL AND PERFORMED CHECKS USING AIRCRAFT MAINTENANCE MANUAL. NO LEAKS NOTED. AIRCRAFT RELEASED FOR SERVICE.