On the tear down of my engine about 5,500 miles ago I noticed that one of the plastic retaining clips that go through the rocker shaft was missing. Since my engine has been put back together I've noticed a tapping sound on the same side as the clip was missing from. This tap sounds like a loose valve that was adjustable would sound like. I read some other forums and now I'm finding that 05 6.0 have had lifter failures. Please tell me this isn't a early sign of it happening.