I think I can here ya KNOCKING!

You may be able to use a broom handle or some solid wood rod to more closely ID where the noise is coming from; place one end firmly on the block or where ever while running, and put the other end up to your ear and listen. Move the rod around to locate. If the flex plate, it will show up at the back of the block or on the trans bell housing the loudest. A rod could be anywhere.