The internal combustion engine is basically a series of explosions, and it's the harnessed power from these explosions which makes the vehicle move.
As far as I can tell, the Harley is the only machine running which actually sounds like explosions. They even market it's distinctive spluttery bangs as a safety feature, along the lines of "You can always hear one coming!" (Of course there have been other marketing ploys too - see above). All other cars and bikes - no matter how loud - sound like other things - a whine, a shriek, a growl, a howl, a sewing machine, etc. etc. The Harley deliberately sounds like a machine shot-gun.
Cars have been designed to get more and more quiet - even the clock in a Rolls Royce is quartz quiet - EXCEPT for the central locking systems. We need to hear a heavy ker-chunk as our car locks itself up with a remote, and designers have deliberately kept it nice and loud, or replaced it with annoying bleeps and flashing indicators, like some sort of servile salute as you walk away.
A friend of mine has just bought a big, electric motorcycle. It's very quiet. Too quiet. He crept up on me the other day in the countryside, and he had to sound the horn before I knew he was there.
I don't think he has ridden it through a busy, pedestrian town yet, but it's only a matter of time....