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Not sure if it is piston slap, since you seem to imply it is happening all the time and piston slap usually disapears as the piston heats and expands and fits the cylinder.
It could be a dirty top engine that someone menetioned; so here is a good cleaning procedure, short of removing the heads.

Option 1.
Remove the spark plugs, pour in some GM top engine cleaner or Chevron Techron, insert plugs FINGER TIGHT, let it sit over night DO NOT START ENGINE.
Next day, REMOVE PLUGS, crank over engine a few times to clear liquid out of cylinders. Install plugs, and start the engine.

Option 2
Purchase a few cans of GM top engine cleaner or Chevron Techron and run it through the fuel system for a few consecutive fill ups.

Option 3
Both 1 and 2.

Would love to see you purchase GM top engine cleaner it helps my company; however, to be honest, its main ingredient is Techron by Chevron. Just make sure to get the high concentration of Techron containers at a good auto store.

Now, how is your A/C operation?
Slugging of the A/C lines can also cause a sound like a rattle, usually heard under heavy acceleration, but can sometime happen under an idle condition.

Can you recognize the area it is actually coming from with the hood open?