I thought I'd post this fix for anyone else who has this issue. The oem ski rack and cross bars whistle very loud at 50-65mph. It is especially bad if you have the panoramic sunroof. I first tried filling in the exposed mounting track on the crossbars by cutting the oem top strip and adding it to either side of the ski rack. This did not fix the problem as the whistling was also caused by wind going in between the crossbars and ski rack. I removed the racks and added flat automotive weather stripping (the kind used between a truck bed and shell) to the bottom of the ski rack to fill in the space between the racks and the crossbars. This fixed the problem completely.