A few days ago I’ve received a response directly from BMW (not a dealership). Here’s what they said:
“ This is a common phenomenon. It’s a slight resonance in the drivetrain, caused by the uneven rotational motion of the driveshaft. It should diminish after lowering the suspension. It’s necessary to adjust the suspension to the current load. In fact, this is a side effect of the Cardan (shaft-drive) design. It usually occurs only on specific gears, as it is a resonance within the entire drivetrain. It’s purely an acoustic effect and doesn’t require any repair action.”
So I guess I’ll continue ignoring it, maybe will try playing with ESA preload, if it even available on my RT…
Thanks everyone for your suggestions!