My benz too was experiencing the rattling just as you are describing. Check underneath your car for cracks along the exhaust piping. Sometime that gets old and brittle and can cause your car to "rattle". When the guys talk about the heat shield, they are talking about the little plate that comes between your catylitic converter and your car's body. This piece of metal protects your car from the cat. converter's immense heat.
So do check out any brittle piping. A simple weild may temporary fix it, but in the long run, the whole piping may need to be fixed if you absolutely cannot stand the sound.
Hope this helps.