'02 trailblazer engine noise 4.2 vortec

My son has a '02 trailblazer with the 4.2 vortec. It sounds as if a camshaft is broke, or maybe he dropped a valve. It is very loud inside the oil filler neck when I listened through a hose, and not near as loud through the dipstick tube The engine seems to run ok, and he says the noise happened quite suddenly on Sunday. I read that this has a dual overhead cam with variable valve timing. Sounds like a real mess to work on, even for an experienced mechanic like me. Has anyone gotten into one of these monsters, or experienced a similar problem? Any ideas or helpful advice? Thanks, Mike S.