Resonance... What a strange phenomenon on bikes.
When I had my Versys some years ago I installed a SW-Motech Crashbar and after that, at certain RPM the bike begin to vibrate... a lot... then I discover resonance in my bike at those RPMs. As I was getting mad because of that vibrations I changed it by another unit from Hepco & Becker and the problem disappeared. Apparently the different design did not cause such as troublesome vibrations.
I recently installed my SW-Motech topcase in my VStrom and now I can perceive the same strange phenomenon. A certain RPMs, the topcase starts to vibrate creating a very deep sound, as if the bike had an aftermarket exhaust (but not vibrations on the handlebar or other place) ... I hope it that sounds diminishes when I put some clothes inside...!