Try unbolting the convertor and shoving it out of the way, then start it up to isolate the transmission totally and see if it still makes the noise. Also, are you 100% sure you don't have a bent pushrod in the valve train? That's what is sounds like to me. Take all the lifter loose and pull the rods and see if any of them don't roll straight. I've heard that noise before and it wasn't pretty.