Still my best bet would be that it is tires/wheels related - sometimes the noise changes slightly with the change of asphalt type (same as Claus noticed). Maybe the soundproofing of the wheel arches? Maybe it comes from the back and it is specific to the touring body?
Are you able to check this issue with the dealer on some other G21? (I cannot do that at the moment, and my dealer will probably say just "meh, that's normal"). Maybe that's just a design flaw with G21?
Or maybe the problem is with our sensitive ears
My passengers generally don't pick up this noise by themselves unless I specifically tell them what to listen to, only then they generally confirm.