Kate,you're spot on about the loud Harleys (and their clones.) I've ridden mostly stock-exhaust bikes through the years, currently have a new Kaw ZR7s and a 75 CB400F and a 78 SR500 thumper. I too wish I could figure out a way to safely carry a guitar but I don't think it can be done.
I've ridden mostly hondas through the years, favorites being a 74 XL250 and my old 76 400F with yoshimura kit... had a BMW R65 for two years but never quite learned to like it.
Guest, there are noise levels here in most states as part of the state inspection, btw, but usually the guys put their baffles in for the safety inspection (kind of like your MOT) but pull them back out when they get home. Many are extremely offensive, and I don't know what kind you guys are listening to, but an unbaffled or "off-road only" piped Hog is plain and simple, way louder than any stock car I can think of in pure dB terms. It's really a blight on the public image of motorcycling. I enjoy a "throaty rumble," btw- the old Triumphs used to have gorgeous sounding exhausts- but a lot of these are just plain nasty sounding these days!