What was found
LOUD NOISE COMING FROM AFT PASSENGER DOOR. FLIGHT ATTENDANT REPORTED BEING LIGHT HEADED. NO INDICATION IN COCKPIT OF PRESSURIZATION ISSUE OR ANY CAUTION OR WARNING MESSAGE. RETURNED TO FIELD. LANDING UNEVENTFUL. REMOVED FROM SERVICE. PERFORMED GVI OF AFT PASSENGER DOOR SEAL AND SURROUND. FOUND CONTAMINATION OF SEAL WITH DE-ICE FLUID. PRESSURIZED/ DE-PRESSURIZED AIRCRAFT IAW AMM 21-00-00-863-801/802 NO ABNORMAL NOISE NOTED FROM SEAL WHILE PRESSURIZED. RETURNED TO SERVICE. NO PARTS REPLACED.