The noise is most likely coming from the cam timing chain tensioners.

We/I have done a couple at our dealership.

The 4.0 and 3.5 have 3 timing chains, a main chain and 2 sub chains that drive the cams, each chain has its own hydraulic chain tensioner, so 3 tensioners also.

What is happening is the tensioners that hold the cams chains tight are wearing the teflon surface off, as the teflon wears away the steel plunger of the tensioner comes in contact with the surface of the chain, and their in lies the problem.