there is several things it could be:

- timing chain rattle
- blowing exhaust manifold gasket
- cracked exhaust manifold
- or it could be the "tick of death" meaning the head gasket and bottom end are on its way (which is quite common with these engines).

hopefully it isnt a serious prob, but if it is you are better off swapping a second hand engine in becuase they cost silly money to strip down and refurb. as a guideline i know someone who did the headgaskets on a duratec v6 himself, and it cost over £800 in parts, head skims etc