I have exactly the same problem with a Crane in mine. It doesn't seem to noticably affect performance, just very annoying. Switched rockers from H.S. to Isky Iron. No difference. Tried all different preloads. No difference. Tried different length pushrods. No difference. Looked to see if rockers were hitting valve covers. Nope. Thought maybe what I really had was a header leak so I changed to Remflex gaskets. No difference. I give up. I'll put a solid in it before I try anything else. Then it will make less noise and have more power. Perfect.

In theory, there is no difference between theory and practice. In practice, there is.