I recognize that noise. My 23 did it, my 24 did it until around 15k miles then stopped, and my wife's 24 Yukon does the exact same thing and hasn't stopped. Sometimes it does it cold, sometimes it does it hot. Never in time with the engine just very intermittent. I've heard it after driving on the interstate for an hour or warming up in the driveway. I've wondered if it's the exhaust manifold or heat shields being affected by heat because it's not a lifter. It's a very hollow sounding almost crack noise, not a knock.