I'd suspect something in the carbies making the noise but without fiddling around with it and a piece of diagnostic garden hose it's hard to guess. Having a dyno so you could run it up and then get around it with the hose to pin point the source would be very useful. I'm sure someone on this forum has come across it before...

My thinking is that there might be some kind of restriction or resonance going on there or something's not quite tight. COuld even be a piece of gasket partially obscuring a passage in the carb making the noise at a particular speed. You'll figure it out I'm sure.