Although the tire tread is most likely the root cause of the vibration, it is quite possible that the howl itself is resonating from another component due to the harmonic vibration. Another words it might actually be the belt guards or maybe the rear fender that resonates the sound making it audible only at a certain vibration frequency. That might explain why people are noticing the howl at different speeds. No two machines are truly identical. If that component can be singled out, it would be possible to dampen the vibration thru insulated mounts. I had a car many years ago that had a similar issue and it ended up being the heat shield for the catalytic converter. It would sing out with a weird sound that made you think it was possessed by demons. Once I removed the heat shield the issue went away. So changing the tires might might stop the howl, but it might simply be due to the new tires having a different vibration frequency, thus eliminating the howl. Or a new set of tires might simply howl under a different scenario. Faster, slower, higher revs, lower revs. Changing any variable will change the harmonic frequency.