What was found
ON CLIMB OUT FROM TPA, A VERY LOUD SQUEAL WAS COMING FROM THE FORWARD ENTRY DOOR. AC WAS PRESSURIZING, NOISE BECAME LOUDER AS AC CLIMBED HIGHER. FLIGHT RETRUNED TO FIELD AND LANDED WITHOUT FURTHER INCIDENT. NO EMERGENCY WAS DECLARED.MAINTENANCE REMOVED AND REPLACED LOWER HINGE FLAP ASSEMBLY COVER ON FORWARD ENTRY DOOR. PRESSURIZED AC TO 6.0 PSI USING ENGINE AIR SOURCE. NO LEAKS OR WHISTLE SOUNDS NOTED FROM FORWARD ENTRY DOOR. AC OK FOR SERVICE.