Seems to me that it is a combination of things: foremost in my mind is the sound from a stock exhaust, has a sort of whistle in it, then the cooling fan and tin gives a unique sound to the running engine. Also, even a stock V-Dub has a mild "lope" to its timing, and one running open flowing headers and straight through mufflers has a sound reminiscent of a V-Twin in my estimation. And, yep, I'm one of those who senses the differences in engine sounds; sometimes I'll perform feats of amazing agility, for a broken, old guy, just to get outside to see what is going down the street or flying overhead.