... injectors? No kidding! I don't disbelieve the possibility, but I've never heard of fuel injectors being so noisy, though that may just be my lack of familiarity with modern engines shining through. I'm only 30, but most of my cars have been older than I am (mostly Corvettes).

I've heard that most 'stick it in the gas tank and drive like hell' fuel injector cleaners aren't even that good, and I have a hard time imagining that anything short of a professional cleaning would help if the noise really is due to dirty injetors, but again, that could be my lack of experience with modern engine blocks doing the talking.

I think I'll take the suggestions of valve train noise, exhaust manifold leak, and noisy injectors to the GMC dealership when it goes in on Wednesday, and see what they come up with. If anyone has anything else to add, let me know! Thanks!