I had the same issue with Thule square bars on my Subaru. The bars made all sorts of whistling racket when they were empty above 50 mph. I used a small 1/4" rope wrapped spirally around the bars about 3" apart for a test and that eliminated the noise. Then switched to 1/8" shock cord I ordered from Amazon with bowline knot loops at each end and they always stay taut. Whisper quiet!