Knock due to what??

I have a 2002 GMC Yukon. It is still under warranty and I've taken it back for a cold engine knock when it is first started. It did not start until after my last oil change. The knock will go away after the engine has been running for 30seconds to a minute. The oil pressure is good.
The dealership that I took it back to twice now, says it is typical and is due to carbon build up on the pistons. My question is this: if it was due to carbon build up on the pistons, wouldn't it continue and not stop after a minute?
What else could it be? And is the dealership trying to avoid a big bill because my truck is still under warranty? Thanks in advance for your comments.