As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AFTER DEPARTURE, LOUD NOISE WAS COMING FROM THE L1 DOOR. UPON INSPECTING THE DOOR, COULD SEE DAYLIGHT BETWEEN THE DOOR AND FUSELAGE. GROUNDED - YES. AIR TURNBACK - REMOVED AND REPLACED THE FWD ENTRY DOOR FLAPPER SEAL IAW B737-800 AMM 52-11-00-020-801 AND AMM 52-11-00-420-801.