Help diagnose knocking issue VIDEO

thanks for the great replies guys. Ok I tried the pull one plug out, each time I heard the engine drop a little, then I would go rev it to that point where the noise comes in. It was the same for each plug.
I dont get the knocking when at idle. It is only as the engine gets up to a certain RPM, then it gets real loud, then if I keep going past it, the sound goes away.

the oil was pretty dark and thick. sounds loudest at the rear of the engine, toward the bottom end.
My compression test seems to show #5 is low 100? all the rest are between 120 - 135.

I am still not real sure what any of this means, except I have begun to price a complete engine rebuild, which seems to be costing about the same amount I just paid for the car 8 weeks ago. sigh....
ideas?