Adam,
When you did your searches, you probably realized there are several things that can be wrong when you get the "diesel sound".
Also in my experience, denial is the common tactics among many a car dealer. There is always a fine line between what should be fixed under the warranty, what might be fixed under warranty, and what should not be fixed under warranty. I think the dealers are monitored by BMW and if they do too much warranty work, itīs not appreciated by BMW. So if they can convince the customer all is well, the noise is normal, donīt worry etc perhaps nothing will break until after the warranty has expired.
(...being paranoid is such fun, you know...)
On the other hand, it is normal for cars to get noisier as time goes by.
Something just occured to me: have you noticed incresed noise from the road, especially when it is raining, too?
If so it might just be the rubber door-seals which have gone a bit dry, thus letting more noise into the passenger compartment. In that case the fix is to rub a moderate amount of BMW "Gummipflege" into the door-seals once a month or so. That stuff would cost you some five quid for a tube that will last you a year or so. Gives an amazing reduction in passenger compartment acoustic noise level.
How to do it:
1. Clean the rubber seals carefully
2. Dry well
3. Rub Gummipflege onto/into the rubber seals carefully, both on the visible sides as well as the part normally not seen of each rubber seal. You need to rub it in a bit (gently), not just smear it on. Donīt forget the trunk and bonnet seals.
4. Let it sit a while with doors open
5. Let it sit overnight with doors closed
6. Wipe off excess from seals and where the seals meet the body when the doors are closed
7. Repeat steps 5-6 for a day or two
Another thing which might help reduce noise is to have the door locking mechanisms (striker plates) adjusted.
When they get everything just right the 5-series are quite quiet cars on the inside.
Anyway, as a general rule, with any car problem: if you arnīt pleased with one dealerīs service department, get a second opinion somewhere else. Just donīt mention youīve already been at the first dealer.
Best Regs,
David