Hi,
I had a similar thing with my mondeo tdci diesel, sounded like a racing car a certain revs. It was an exhaust baffle that was faulty or loose, and at a certain amount of revs it was rattling in time to the engine revs. it didn't do this in all gears, but we eventually found it was when you accelerated slowly through the 2000 to 2,600 rev range that it did it. if you accelerated fast through the range then it never had time to start the throaty roar.
I hope this helps, I may be totally wrong, but its worth a mention, oh, and when it was cold it did it more, perhaps because it was going slower through the rev range.
doris.