As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
RECEIVED SERVICE DOOR CAUTION MESSAGE ON CLIMB OUT AT 20,000 FEET AND THEN LOUD HISSING. PRECEDED BY "THUMP" COMING FROM SERVICE DOOR AREA. PRESSURIZATION REMAINED NORMAL HOWEVER HISSING WAS INTOLERABLE. C/W QRH, DECLARED EMERGENCY, RETURNED PHL. LANDING WAS SMOOTH AND UNEVENTFUL, APPROXIMATELY 200 FOOT PER MINUTE. BRAKING NORMAL.