What was found
AFTER TAKEOFF, F/A NOTICED LOUD AIR RUSHING FROM BEHIND LAV MIRROR, CAPT AND F/O VERIFIED AND TURNED OFF EACH PACK SEPARATELY, AND TRIED APU BLEED, NOISE CONTINUED. NOTICED WHILE AIRCRAFT WAS PRESSURIZED, FOUND AIR COMING FROM WATER TANK, FOUND CAP THAT ATTACHES TO NIPPLE CAME UNCONNECTED. RECONNECTED CAP AND OPS CHECK GOOD.