The sort of vibration you appear to be desribing does sound like its being generated from an 'unbalanced' rotating source. Gearbox initiated, R or D, would indicate that the 'box/TC is involved.
Wonder if you are getting partial drain out from the TC, used to be quite common on some 5-speed autos *.
Do you have a slight delay when you go to pull off? Different from a normal warm start, where it should instantly give positive drive.
A check to see if it is alternator induced, (once the revs change as you engage R or D), just load the alternator with lights and rear screen, to see if that changes the sound track, while still in park.
* I've had my 330d give a vibration from my gearbox/TC, on several occasions, after not using the car a couple of days. Occurs when D is engaged, slow to pick up drive, by normal standards, but it disappears in a second or so. I've suspected that is a bit of TC drain back.