Right rear rattle/grind
 

When turning right only, it sounds lke a coffee can with rocks rolling around under the back seat at the passenger axel wheel, when driving straight, the sounds stops.

Noise from rear
 

Have a look at your rear flex disc at the diff, Mercedes version of the u joint. I have a 124 body, and when they first start "going out" it always sounded like the noise you describe, and I also thought it was from in the right rear. You may also hear some noise when you shift into gear.

Just a thought,

Rickie