This is known as the "dead body in the trunk" sound. A number of owner's have reported this sound particularly on the W211 chassis. Kent explains what he did to fix it in this 2004 E500 sedan. The same problem can happen on other models as well, even without the airmatic suspension.
It could be a number of things including shock absorbers
W110 W111 W112 Sedan
W111 W112 Coupe