Low harmonic vibration??? - E320

[Big Ron]

My 1997 E320 is developing a low harmonic type vibration which is driving my wife insane! I can't tell where it is coming from, but it goes vroom - vroom - vroom according to my wife's description. The vibration can be felt throuh the seat and steering wheel, but I can't tell if it is coming from the wheels, suspension, engine, transmission, driveshaft, rearend, or anywhere. There is a definite low pitched intermittent hum, that someone (other than my wife) may not even notice, but it is there. If anyone has experienced this phenomena, I would certainly appreciate any input. Thank you very kindly. Big Ron