Engine rattling at start up

I have a 02 Avalanche 120K miles

when I first start the truck I can hear what sounds like the rocker arms rattling, after a few mins at idle this goes away, or if I rev the engine, it goes away. Once Im driving the truck, I no longer hear it even at idle.

now that I think about it if Im cruising down the rd, and I stomp on it, I can hear the rattle a little

what do you guys think this could be?