As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
LEVELED OFF AT 23,000 FEET, HEARD LOUD RUMBLE SOUND, FOLLOWED BY STANDBY FAIL LIGHT AND OXYGEN LIGHT. DESCENDED AND RETURNED TO ORD. FOUND FORWARD CARGO SEAL BLOWN OUT OF CHANNEL, DUE TO LOWER DOOR THRESHOLD SILL NOT PROPERLY ADJUSTED. REPLACED DOOR SEAL AND ADJUSTED THRESHOLD PLATE.