I have something that sounds like rod knock on my '04 Chevy Silverado with a 4.8L V-8. The reason I think it's not really rod knock is cause it's worst when it's really cold outside (think sub-zero with no block heater) and it almost entirely disappears once engine has warmed up. What else could it be?
I'm going to go out on a limb and suggest what you're hearing is piston slap. The earlier LSx (4.8, 5.3, 6.0) engines in the trucks had this issue. I have "heard" it stems from the oil control rings getting stuck in their lands, but don't know the efficacy of that information. It is also my understanding that, while it is really annoying, it won't cause your engine any issue.
As an aside, rod knock will not go away once the engine is warmed up. Usually it gets worse after everything is warmed up because the oil flows better, and squishes out easier.