I had a Tempest with a 400 in it. The water pump has a stamped sheet metal plate in it. That plate came loose in the one I had and sounded exactly like that. Í know you ran it without belts but the noise I heard in the video is what my Tempest sounded like with that loose plate.
Jerry
I had a 80s Fox Body with a 5.0 the water pump shaft "walked back" and rubbed the metal plate on the back of the water pump. That is a very possible occurrence. id be stethoscoping it and trying to pinpoint.