My son does a lot of motors we also had two sets of comp lifters clatter went away and always came back. Depending on where the motor stopped and sat. Comp had a bad lifter problem for a while by their own admission. We had a old 474 cam kit here from "Direct Connection". We installed those lifters and problem was gone. Pontiac uses the same lifter and same problem.
Install new lifters and check the surface for crown on all you pull out. If the crown is gone from clatter you'll also need a new cam now. Comp should take care of the replacement they did for our engine shop. They would not pay Labor, gaskets or any material. such as oil or Zinc
