First BMW I bought was my 1100GS and immeditaley noticed the drone in 5th gear. Being the paranoid type I stripped the final drive enough to examine the bearing, found no issues and put it back together. It's now been droning for another 50k with no issues. Changed a drive shaft 5k ago but drone still there so it's not that. Just some type of harmonic vibration I guess.
1974 Laverda 750 SF2
1974 BMW R90S