Yikes, a '94 is an "older" Benz? What does that make my two '72's?!
Rattles while moving are usually things like loose strut mounts or something else associated with the wheels, although you could get something that rattles only when the engine is rotated from torque.
I'd check the condition of the engine mounts -- they should be about an inch higher than the frame a the widest part. Also, look for something loose -- a broken serpentine belt tensioner comes to mind, along with a bad belt tensioner shock.
I believe there are some plastic covers in there, too, so one of them may have a clip or screw hole broken off, allowing them to rattle when the car is in gear when vibration is higher.
Eventually you are going to have to poke around and look. Most likely a broken clamp or holder, or something coming loose.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!