I really enjoy driving my CLS. I do remember though the speakers would rattle, the doors would rattle; I drove over a rough patch of road and I thought to myself, "why did I trade in my avalon." That is how bad the speakers and rattle were.
Slowly, piece by piece, door by door, I began to dynamat everything that rattled. Dynamatting the front doors (and adding new woofers) paid out huge and completely stopped rattling that used to occur by the front arm rest.
I got to a point though when I didn't know what to do next. I turned to the mbworld for suggestions. I sound treated the rear grill, and later the area where the back up indicator lights are. Both suggestions of mbworld.
This article deals with the area in the rear where the indicator lights are. It is detached from the roof by pulling down by the glass, popping a clip on each side with straight downward force. No pry tool is needed. Then this slides forward to release the part from the ceiling.
Inside you'll find a plastic part that is prone to resonate rattling, by this I mean if you tap it, it sounds hallow and perhaps a bit shaky. There is a soft noise deadener applied, carefully remove this part, apply the dynamat beneath it, and reattach the soft noise deadener with the dynamat backing to use as glue against the soft deadener.
I applied dynamat to this plastic part and to the areas that hold the clips in place. Be careful not to plug the holes though. as their are fingers which slide through these holes. I added a bit of dynamat above this plastic piece to the bare metal areas. I tapped on each to see which "rang" or were hallow sounding and applied to those pieces.