As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
CREW REPORTED A LOUD -HISSING- SOUND COMING FROM THE MAIN CABIN DOOR ON CLIMB. AS DELTA P INCREASED THE SOUND INCREASED. THE AIRCRAFT RETURNED TO THE FIELD WITHOUT INCIDENT. NO EMERGENCY WAS DECLARED. MAINTENANCE VISUALLY INSPECTED THE MAIN CABIN DOOR SEAL. NO DEFECTS NOTED. OPERATIONAL CHECKS WERE PERFORMED. NO DEFECTS NOTED. OPS CHECKS GOOD. AIRCRAFT WAS RELEASED FOR SERVICE.