'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.
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There have been issues with cylinder sleeves cracking noise from the piston area, some of these trucks were sent extended engine warrantees, if no misfire obviously it can be a broken cam or dropped valve, pull the v/c off see if it is in the valve train, tough engine to work on, possible pulled a valve thru the retainer and the noise is just starting.

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Thanks for the input, I am getting ready to tear into it this morning, will keep you posted as to the results. Mike S.
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