jrmurray,
Tried everything with mine that has been suggested above.. loose parts, tire noise, etc, etc.
I gave up and took it to the tech I use who used to work at an MB dealer. He said the only thing left was to change a wheel bearing. The old bearing looked brand new when it came out
Noise never went away. Stayed exactly the same for the next 6 months.
I got upset and took it into my own hands. First thing I did was to drain & refill the rear differential fluid with full-synthetic (not sure of the brand, but I think it was Mobil 1)
Problem disappeared. I have now put 40,000km (25,000miles) on the car over the last 1.5 years and the problem never came back. It's dead silent now!
Change the fluid first! If it works, wait a few months and change it again to make sure all of the old stuff is out, and you should be fine.
Neal