> I think the thing with motorbikes is everything is so much more exposed, and they rev higher in general, so the sound thing is probably a harder problem.

Meh. Some have figured a way to make them quiet. Most "touring" bikes are very reasonable. I'm thinking BMW K16s, Honda Goldwings, Yamaha FJRs. With stock pipes, of course. Hell, even my VFR1200 is very quiet with the stock pipe, which closes a flap below 5000 RPM. When idling at a traffic light, if there's a car around, I can't hear the bike.