My friend has the same issue on his 04, we suspect either piston slapping or possibly a lifter. The truth is, with an engine, it is really hard to tell, and a dealership that cannot be trusted is likely to use some lame excuse like piston slapping or lifters, without diagnosing the problem.
If I were you I would take it to a trusted mechanic, because here we are only speculating based on sound bytes and written evidence, which is not going to give you the real answer that you are seeking. If your problem is serious I would feel awful staring you in the werong direction.
If the car is under warranty, I think GM will come up with any lame excuse in the book to get out of fixing it. Being that we live in a capitolistic society, this reasoning is quite logical.
IMO I think you will be fine as my friend has 54k miles on his goat, and has the same issue, but make sure you do what you need to do to sleep easy my friend!
Good Luck and Godspeed! Lettuce know how things work out!