The water is probably coming in from a leak in the roof or around the eaves. Look from outside for holes and rotted wood. Look in the attic for water staining on the planks that make up your roof structure, then follow the staining back to the hole that is letting the water in. If that doesn't work, have somebody run a hose on the area of the roof near the leak while you are in the attic and watch for water. Roofs leak a lot more often than actually having a leak in the wall and water can "travel" easily by running along the framing in your attic.