Hey everyone.

I have a 2003 silverado 4.8l 2WD
Has 300,000km - so about 180,000miles

Just recently started it up and it has this noise, like rocks bouncing around in a tin can.
First I thought it was a rock in the heat shield of the CAT, but then I got a check engine light now. It is coming more so from under the engine somewhere, rather then the top of the hood.

The piston slap I know of, and goes away after a few minutes. This sound on cold dtart is loud, but gets slightly quieter as I drive, and it warms up? But is still noticeable?

Enging runs fine, I have all power still. Good oil pressure at 275kpa, Nothing that is different with the engine except this rock noise.