Having issue with wind created by your roof top bar?
Very slight adjustments to the angle of the bar can make a big difference in sound. Please try multiple slight adjustments at varying speeds to find what is best suited for your Bronco. Also be sure that your bar is tightly secured.
If no improvement is found via above methods, you can also attempt adding edge trim (available at local advance auto parts etc) to the underside foils as shown below. This has been shown to be an effective method.