Did you ever get the rattle fixed? I am seriously considering a '94 E420 with 55,000 miles for sale at an MB dealership right now. See the ever-growing thread in "Featured Cars" called "E420 Horror Stories".
This car has that same rattle, or tapping sound over bumps. Also, with the car moving or sitting still, you can slighly move the steering wheel back and forth and generate the same sound. It's kind of like turning a flat head screw driver in a slot that's too wide. It taps the sides as you rotate back and forth. Any ideas?
The sales guy said he rode with a technician to listen to the noise. Technician said, "It's probably nothing to worry about, but we'll look at it if you want us to." Darn right I want you to!
1980 450SL @ 103,000 miles
1966 230SL @ Unknown Milage
2006 C230 @ 110,000 miles (Sold)
1994 E420 @ 118,000 miles (Sold)