Check your steering rack - mine is doing it too. Sounds almost like a bad ball joint, but isn't. With both front wheels sitting on ramps, have someone gently wiggle the steering wheel back and forth, then put your hand on the rack right by the input shaft. If it's your rack, you'll know right away because you'll "feel" it. It's only apparent if you move the steering wheel ever so slightly, maybe just a half inch back and forth, enough to wiggle the pinion gear. I can't think of a better way to describe it. I discovered this while inspecting the tie rods, ball joints, and other suspension parts; my wife was moving the steering wheel. I was stumped because everything was tight, no unusual tire wear, steering was not sloppy, etc. but then I stumbled upon this and was able to duplicate it quite easily.

Mine makes the noise occasionally, but there's not enough lash in the steering wheel to warrant replacing it. When I looked into it, I believe I read somewhere that Mercedes won't replace a rack under warranty until there's about 1 inch of play in the steering wheel??!! I just put up with it as an occasional light clunk when going over very bumpy roads. None of my passengers can even tell. And if I do ever decide to replace the rack, I've priced it, and it's not expensive. About $300 to $400 for a reman, and a few hours on a rainy day...

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