Sometimes when I have stopped and sitting with my foot on the brakes or just idling I hear a sound that I can only describe as a momentary chain rattling? Any ideas?

 

My rear muffler makes that sound. But where do you think the sound comes from, does it chnage if you are parked and rev up slightly; how fast do you need to be moving for it to go away?

 

We need more information then that, get under the hood and listen closer.
If it was the chain, and at your mileage, it is getting sloppy. If you feel it is coming from the front cover area, then buy a new chain and gear set along with the oil pump repair kit and go for it before it fails.

 

It is more of an electronic 'GRRRRRRRRRRRR" sound if that makes any sense and it seems to be from the front of the disco.

 

Do you still have the factory stereo or was it replaced? If it was replaced, maybe a ground-loop problem?

 

The disco has all the original equipment and has been maintained by the book at the dealership.

 

It is your SLABS unit, that is the ABS modulator (as well as a few other things) that is making the noise, there is a TSB on it.
After you shut your truck off go back out 10-15 min later and see if you can still hear the noise from under the hood.
If you were to let the truck sit for a week undriven the battery would go dead.
Take it to your dealer and have them check the TSB status of your truck.