This may sound silly, but as you were assembling these, did you go through at least 2-3 cycles of crank rotation? I wonder if the pushords happen to be hitting the head in the head passege somehow?

I did have this happen...I knew the fit was tight, but thought it was good enough. Not quite...after the initial start-up, I pulled the covers to find a good/deep scuff mark on one of the pushrods...it was enough to give it a good clatter though.