As has been suggested already just crack open one of the rear windows by an inch or so which will stop the boominess. The speed at which this happens depends on the wind speed outside the car. It's never less than about 50mph in my experience but sometimes if the air is particularly still I can drive along at any speed without boominess.

Another option is just to open one window in the roof, this shouldn't cause any boominess. You can also press the button which will cause both roof panes to tilt and provide air without any undue noise.

If it's any consolation with the roof fully open the boominess completely stops when you drive about 105mph...

__________________