My 74 two bolt 400 SBC developed a severe knock over the past couple of days. Could only be heard at 3k+ rpm but then moved quickly into the lower ranges until it could be heard at idle. Sounds like it is coming from the mid center or top center portion of the engine although it is hard to tell. Video link below shows that it comes and goes which is what I can't exactly figure out. I'd like to have a better idea for a diagnosis before I just break into it. Any theories?

http://www.youtube.com/watch?v=IleBauFOnS4