So I noticed this slight rattle coming from the passenger side, but I couldn't pin point where it was coming from. I was driving on the highway and I had my Distronic ON. I decided to press my hand against the passenger headrest and the rattle stopped. I took my hand off it just to make sure and sure enough the rattle returned.

First I tried extending the headrest, but that didn't stop the rattle. Then I retracted the headrest, but that didn't work either. I finally reached all the way over to the passenger seat control and lowered the headrest all the way down and that's what made the rattle stop.

So if you have a rattle try lowering the passenger headrest.