Wind noise can come from light bars when they are mounted in such a way that wind can blow across the cooling fins on the back of the bar, causing noisy vibrations. This is most common on Jeep Wranglers, as the shape of their windscreen makes this a common problem.
The best way to get rid of this problem if you must mount your light bar in this position is to purchase some flyscreen spline from a hardware store and press it into the gaps between the fins on the back of the bar.
You will need to run the flyscreen spline along the entire length of the heat-sink. In most cases it is only necessary to insert the spline along the bottom half of the light bar fins.
This will seal off the gaps, and prevent wind from being able to vibrate the bar.