Stops as soon as you let go of the accelerator and only above a certain speed? If so it may be the propshaftHi, I have been struggling this problem.
Does anyone have similar experience?
You can see the problem through following Youtube link.
If ever you find the cause, please post it here. I've been to innumerable Dealers and "Experts" to no avail.
I have a 2004 E46 320d with that vibration for the last 5 years and have had to be fatalistic and just learned to live with it.
I have replaced or overhauled the following:
Torque Converter twice.
Prop shaft, 3 replaced
Prop Shaft Flexible coupling
Prop Shaft Centre Bearing
Engine, Gearbox and Diff mounts.
Overhauled both side shafts.
Re-trued rear wheels.
etc etc etc.
I've read somewhere that some 320d's have a Harmonic Vibration which is normally detected by the factory, after which Vibration Dampers are fitted either side of and just behind the Diff.
Another possibility which I haven't investigated yet, is that the gearbox input and output shaft Speed Sensors get out of sync somehow, causing the box to be undecided about changing gears, thereby setting up a vibration in the drive train.
It's been an absolute nightmare!!
Good luck. You're going to need it I think.