Hey Tex,
It seems to me you only have a few things to check by process of elimination. It's either a bad lifter, a bent push rod, or a bad rocker arm. There is a test you can do to see if the noise goes away by tightening one rocker at a time. If the noise stops you found your problem provided that the correct lash was set on all the lifters before you started. Chances are the one making the noise will get real quiet when you tighten her down. That's the one I would replace. Push rod and lifter.
I hope this helps you out.

Crusty