Also just a point, try another bike that has 1,2,4, or 8 cylinders. 3 and 6 cylinder engines share secondary harmonics so if one engine causes a resonance it's highly likely the other configuration will as well.
My bell does not resonate at all, my shoei either, my Nexx does. It's kind of random which do and which don't, but if they do, there's next to nothing you can do to stop it. All 3 are expensive composite shells, all roughly the same external dimensions, all have massive view ports, like I said semi-random.