Don't know what your noise is but it's not piston slap. Piston slap goes away as soon as the engine warms up and does not return until the engine cools down again. You cannot make it slap by hard ***** to the walls acceleration.
I second the thought that it could be ignition pinging.
If so you can try the Seafoam method. Be careful not to add it too fast as you don't want to hydro-lock then engine. I would also put in two cans on a quarter tank and do a few WOT runs.
If you haven't put in a fuel filter in a while then now is the time.
It is also not a bad idea to put on a vacuum gage and make sure you have 16-20 inches vacuum at idle after the engine reaches operating temp. A vacuum leak can make it run lean and ping like hell.

 

My dads 2000 with the 5.4 will make a rattle noise while hard accelaration. We were also told it is either a heat shield or even the dash that is rattling, which is very common. Good luck.

 

Beware of flying spark plugs. You may want to check them to be sure they are all tight.