Just a thought..might want to check the wheel bearings in the front..Also, (happen to me once) check the lug nuts... they may have backed loose a bit especially if you have any tire or brake service..But listen to MB DOC and the others..you need to isolate when it is occurring...is only at certain speed.. will it do it in neutral..will it do it with only passenger in there..etc.. Start with the simple first then work your way up to the big stuff...I wish you the best of luck and hope its a cheap fix....