Your problem sounds like the explanation I give at the end of my wright-up.
If it happens at start-up I would suspect a lifter plunger that is getting stuck in the body. If there are fairly new lifters the problem should go away in time as they wear in.
The one out of five times noise is because an engine does not stop at the same spot every time it shuts off(a six cylinder stops in one of three spots). If the engine stops with a sticky lifter holding a valve open, as it bleeds down from valve spring pressure, the plunger will end up being pushed to the bottom of the lifter body. As the lifter cools the plunger becomes trapped in the body. It is not until the lifter body starts to heat and expand that the plunger is released, at this point the lifter starts working properly.
