It isn't timing chain rattle: The Zetec has a timing belt and doesn't use oil pressure for tension.
But there is an idler on the timing belt; it should have been replaced along with the timing belt itself.
I've also seen noises like this come from idler pulleys on the serpentine belt.
Open the hood before you start it the next time, and walk around in front of the car, listening to the noise. You should be able to discover approximately where it's coming from.
Actually, the serpentine idler pulley on my Contour is failing now. Mine shows up as a squeak that goes away as the car warms up. Using the "long-screwdriver-to-my-ear" method, I was able to pinpoint exactly where the noise was coming from.
