I just watched it in slow motion. Looks like a piece of foam came off the tank. You could also see its shadow on the surface of the shuttle. The object has collided with its shadow, which means there was a contact with the surface of the orbiter.Something like that, it's very easy to see, its as bright and large as something like a tile. Then shortly after you see a piece come from the tank and ping off the underbelly of the shuttle
I couldn't catch it closer.. good clip!