Then it's video time. Use your phone and the YouTube app, take a video of the engine bay with the hood open when you start the truck, and film until the noise changes or quiets. Take another short one of the engine at full operating temperature. It's hard to diagnose noises on a forum where we communicate in writing, lol.
If you don't want to do anything else, do an oil change with RedLine 5W-30 and use whatever filter, though I recommend Royal Purple 20-820 or Fram Ultra XG (or Titanium FS2 if you want the extra fancy version). Do 500 to 700 miles and come back here and let us know if it sounds any differently. Try not to be overwhelmed with a placebo. Expensive stuff will do that to you, lol.
Also, I wouldn't run the stock manifolds without shields, which can harm the knock sensors.
If RedLine 5W-30 quiets your HEMI, then you have rattling -out-of-tolerance- lifters, and you can delay replacing them, maybe for a very long time. If it doesn't, you have other problems, possibly more significant than exhaust manifolds. And if that's the case, then it's time to start measuring valve lift.
Old-school marketing at its finest, isn't it?