This is going to sound dumb, but I had a similar sound with my Super8 150 at about 50 mph.
I went through and checked for loose screws and bolts, and couldn't figure it out.
So I cleaned out the under seat storage compartment, so I could take it up to the dealer, and the sound magically disappeared!
Turns out, in my case, it was some of my junk vibrating against the tub under the seat so it sounded like an engine noise.
I don't know if you keep stuff under your seat all the time, but if so, take everything out and see if the noise is still there.
Hope your's turns out to be a simple thing like mine was. Keep us posted if you find the culprit for sure.
Sorry I can't be more help.
Duke-