We had this problem while driving our new-to-us 560 Ultra home from Kentucky where we purchased it. We have a Ford Edge. The whistle drove us crazy! My fix was to buy several feet of black rubber tubing at an auto parts store that was the same size as the tunnel in the rail and, using liquid dishwashing soap as a lubricant and a pair of needle nose pliers as a tool, pull the tubing from one end of the tunnel to the other, completely filling it with the rubber tubing. I then cut off the two ends of the rubber tubing flush with the ends of the rail. No more whistle!
David Pedersen