Mine has the exact same problem, however I just figured it was because of the light bar that I have on the front of the truck.
My Truck
We took a trip to LA (about 6 hours from here) and there were several times I almost pulled over to break the thing off! In the meantime, my wife is giving me the "eye" like, "You wanted a hummer!"..
It almost seems like it needs another support bracket (like the ones on each end) in the middle. I can see it vibrating in the middle.
I am going to take it off (maybe tonight) and see how it is on the highway, as well as in town with the sunroof open. It might be worth the wind comming into the vehicle just to get rid of the noise!
I feel your pain!!!