My brand new 21 makes this noise too. My wife's 04 yukon xl with the 5.3 makes it as well. Same exact scenario, once truck is warm it doesn't do it. The yukon has a lot of miles so I didn't think much about it. I preferred the way that the 6.0 sounded at idle as well. The 6.6 is kinda ticky. I like the truck a lot though