I have just had the third replacement DMF fitted (all under warranty apart from the first new DMF) on a 54 plate 1.8tdci, as each new DMF has started the rattlle you describe within a few thousand miles. The distinctive rattle on engine stopping was the first early warning sign of imminent DMF failure on the origional clutch at 92k which led to total failure within 3k of the first noticable noise. The garage indicated that premature failure of replacement DMFs is common on Ford cars with the 1.8tdci power train. During the first warranty replacement I had the starter motor and glow plugs replaced to eliminate them as a stress on the DMF. A straw poll of other people I have annoyed in supermarket car parks, and also a friend that is an AA roadside assistant tech indicate that this is probably something that Ford are well aware of but seem to have no intention of addressing.

At £500 a go for the spare parts they possibly do not have a motive to do so!

The death rattle is quite distinctive and probably caused by the inner section of the DMF bouncing back and forth a few times on the rubber shock adsorber and hitting the limiter.

Make sure you go back to the garage and register your concerns before the warranty runs out, even if they dismiss the DMF as being the problem. Put your visit in writing and confirm their opinion. If the DMF then completely fails shortly after the end of the warranty you will be in a better position to claim in the small claims court.