What was found
LOUD SQUEAL THROUGH 10,000 FEET. PASSING 13,000 FEET SQUEAL BECAME LOW PITCHED AND CABIN ALTITUDE BEGAN TO CLIMB. DESCENDED, DECLARED AN EMERGENCY AND RETURNED TO FIELD. REMOVED AND REPLACED THE PASSENGER DOOR SEAL IAW AMM, OPS CHECKED GOOD. PERFORMED PRESSURIZATION TEST IAW AMM, PRESSURIZATION TESTED GOOD.