We get a whistling noise on ours also. It starts at around 35-40mph, quietly and subtly at first, then gets louder as you go faster. We can't hear the noise with music playing, but it is slightly annoying at 55-60 without music going.
Just for giggles, I closed the center vent and immediately lost about 75% of the noise. I opened it back up to see if I was just "hearing things" and the noise came back. Closed the vent again, noise greatly diminished again.
My guess is (at least for our bike) that the air moving through the center vent is being forced into it just enough (like a venturi), that it makes a faint windy-whistling kind of a noise.
Try it on your bikes and see if it helps the whistling noise level to close the center vent.