Low Frequency Vibration Sound

[Fifty150hs]

I have a 2006 3.0 M54. When sitting at idle at a light or elsewhere with the car in gear in makes a low frequency vibrating noise that is annoying as hell. I've cranked up the idle speed as high as it will allow and it reduces it but doesn't eliminate it. It is more of a sound than an actual vibration that you can feel. When idling in park it doesn't make the noise. It's driving me nuts. I even put the X on jack stands, started it up, put a brick on the brake and put it in gear to see if I could isolate the noise. No luck beyond it seems like it is louder at the rear of the engine, but I couldn't tell anything more specific than that. Anybody else have the same thing? Were you able to find the cause? Any ideas? Car has 292,000 miles on it.