What was found
WHILE CLIMBING THROUGH FL260, A SUDDEN LOUD HISSING STARTED FROM BENEATH THE F/O'S FEET. LEVELED OFF AT FL270 AND PRESSURIZATION STABILIZED. THE HISSING STOPPED ACCOMPANIED WITH A BUMP IN PRESSURIZATION. THE REMAINDER OF THE FLIGHT WAS UNEVENTFUL. NO EMERGENCY WAS DECLARED. MAINTENANCE INSPECTED THE FWD AVIONICS HATCH SEAL AND CLEANED AS REQUIRED IAW MM 52-43-00. THE NEXT FLIGHT, CLIMBING THROUGH FL240, ENCOUNTERED SAME DISCREPANCY. MAINTENANCE REMOVED AND REPLACED THE FUSELAGE DRAIN FWD OF FWD EBAY DOOR IAW EMB 170 MM 53-05-00. PRESSURIZATION CHECKED GOOD ON GROUND. THE SAME DISCREPANCY OCCURRED ON THE TEST FLIGHT. MAINTENANCE REMOVED AND REPLACED THE FWD AVIONICS COMPARTMENT DOOR SEAL AND THERE HAVE BEEN NO FURTHER EVENTS.