Sounds like exact scenario I had with original motor mounts failing. In park and at speed the vibration and noise would smooth out. So if not the motor mounts, I'd still think its something to do with the engine vibration.

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2003 X5 3.0i Silver (build 07/03)
2006 X5 3.0i Blue
1977 Ferrari 308 GTB
1994 Ferrari 348TS Speciale #14/15
1989 Ferrari Testarossa