What was found
DURING CLIMB OUT THROUGH 8000 TO 9000 FEET, HEARD A LOUD THUD AND AIRFLOW NOISE. AT APPROXIMATELY 13000 FEET, CABIN ALTITUDE EICAS MESSAGE DISPLAYED, FOLLOWED QRC & QRH. EMERGENCY DECLARED, FLIGHT LANDED WITHOUT FUTHER INCIDENT. THE AIRCRAFT WAS GROUNDED. MAINTENANCE FOUND OVERBOARD EXHAUST VALVE INADVERTENTLY DEACTIVATED IN OPEN POSITION. COMPLIED WITH AMM 21-00-00-049-047, DDG PROCEDURE 2 FOR OVERBOARD EXHAUST VALVE DEACTIVATION IN CLOSED POSITION. COMLIED WITH CABIN PRESSURE LEAKAGE TEST IAW AMM 05-51-24-782-001, LEAK RATE WELL WITHIN LIMITS.