Not sure if you ever got enough feedback on that noise, but we have a 2000 ML 320, and there is always a very faint sort of whining sound that seems to come from the transmission or drive train at highway speeds. Pretty sure it is not tire noise. It is a very faint noise; the wife doesn't hear it and the son, whose car it is, says, "oh I've gotten used to it and I don't even notice it. Fortunately this 2000 does not have that rubbing noise that you get when turning the wheels to full lock on some of the earlier models we drove.