As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
LOUD FLAPPING NOISE AT L2 DOOR. INSPECTED DOOR, SEAL AND AND FRAME. NO DISCREPANCIES NOTED AND NO EVIDENCE OF ANY OBJECT FLAPPING ON THE EXTERIOR OF THE FUSELAGE. SUSPECT NOISE IS ASSOCIATED WITH BLOWN APU DUCT THAT CAUSED DEFINING NOISE IN AFT GALLEY. THE BLOWN DUCT IS LOCATED LEFT SIDE JUST BELOW L2 DOOR.