brought my 2003 Exp in to the dealer for the exact problem. They are going to replace the timing chain tensioner(s) under Warranty. The ESP premium warranty really paid off since I think my Exp was built on a Monday... had the rearend replaced twice, rear A/C blend doors, DVD player, radio, rearview mirror motors, and driver power window cluster replaced

 

If you do a search on this site you'll see lots of references to that rattle on the initial startup.. sounds like it's pretty common and no cause for concern.

 

could also be the cat. converter heatshield. I have a 98 with about those miles and it started making alittle rattle when cold but when it heated up and expanded, it would go away. I was able to re-tack weld it back in place and no more rattles

 

The timing chain tensioner replacement took are of the problem.

 

An air ride compressor test for noise? Start the Expedition with the passenger door (or any door) open. A compressor wont run with a door open, then you can isolate the noise as coming from the engine or the are compressor. Once all the doors are closed, the compressor should kick on and it has a different sound than engine rattle.