[W126 Coupe] Odd, speed variable, sound

[Shayegan, Richard]

Something started today, it's very quiet unless I'm driving with the windows open and there is like a car or house or something nearby. It sounds like comes evenly from both sides (i.e. not just the front or right), and it is near the front of the car. The sound is like a steam engine train (you know, chug-chug-chug-chug, that exact train sound), and it will happen engine on or off. The pace varies with the speed of the car. Any ideas?
Thanks,
Richard