Has the boat spent any time on a mooring? She may have soaked water into the stringers. When you have it on the trailer, crank the trailer up as high as it goes and see what comes out of the bilge. If necessary, drill a test hole at the inside bottom of the stringer, at the transom. Drill maybe a 1/2" or 3/4" hole. Also, does the boat have trim tabs on it?