Something I've never heard discussed is the sound a pair of 650s make. I rode a lot with my airplane partner back in the day and the heterodyne they make is very cool. Pulling out to pass on a two lane in echelon at full throttle in 5th sounded so cool. I think that is one the reasons the Harley guys tolerated the 650 riders. They sing real nicely with a sportster. A close second is being in the middle of a gaggle of air- cooled 350 2 stokes around a track. Crazy sounds with the expansion chambers at full song.