I’ve been trying to fix a whistle from a pair of aerodynamic (plane wing shape) cross bars for almost a month now. I tried the rope wrap, taped up all the holes, different positions and nothing worked. Finally I got the idea of putting a strip of fuzzy weather stripping along the bottom edge to brush the air below before meeting the air above the bar. Turns out it works really well! There’s still a very subtle whistle, but it’s amazing how well it works and for how cheap it is. Hope it’ll last. Good luck!