When I put my van in gear there is rattling behind the engine

Edit: Here's a better way of wording my question -

When I shift van into gear (either D, R, L1, L2) but not in neutral and sit in one spot with the brake pedal depressed there is an awful rattling noise. It does not occur in park or neutral with the brake depressed. The rattling seems to be causing drag on the engine as the idle will drop down. It's almost as if every drive pulley is seizing up. This does not happen if I hold brake down while in park or neutral.

Also there seems to be fluid leaking. Possibly transmission fluid.