mine does the same thing, we have about 2 threads a month about it so it seems.
It's called piston slap I think... I have it, and I know every other 3.0 that I've seen and heard has it too. It sucks, but we must deal with it.
You can make it quieter with soudn deadening. Make sure all your rubber gromets on your firewall are pushed in all the way.