Try deflating your tubes. I've had water slosh around there once and it turned out it was just stuck between the tubes. Once I deflated the inner tubes on my FP80 after a rainy race and water ran out. Hope that works!

I had an old set of Spinergy's once that had the decal covering the drain hole. I asked them, they said it's there and about where I would find it, so I prodded around and then poked it open and no more water in wheels.

It was hilarious the first time, heavy rains on the way home, Stopped in town after the rain stopped, and it looked like a fountain coming out of the valve hole. I bet my bike gained about 4 pounds in water weight between both wheels.