I took the car to the Services this afternoon,
The master technician took a ride with me and acknoweldged the booming sound.
He admitted he hadn't heard this kind of sound on any other cars, and while he thought that the sound didn't seem like there is something wrong with the gears or the exhaust, he couldn't say whether it was intended or not, either.
So he says he's gonna refer the case to BMW headquaters for an answer and it might take couple of months
I did get a general software update, but it wasn't particulary about the engine nor the transmission and it did not 'fix' the booming sound.