Valves Clatter

Look at the tips of the rocker arms where they contact the valves (this will require removing them first). The tip should be perfectly flat across the radius - no divot in the center. If there is a divot, then the rockers should be refaced by a qualified machine shop.

While they're out, check the fit of the pins in the rocker arm bushings.

I've seen cases where someone has mixed valve train parts - later pushrods and early adjusters for instance. There is a difference in the radius of the ball and socket ends, which if mixed can lead to excessive noise.