This probably happens because the column of air in the rack is vibrating at a resonant frequency. The vibration is a function of the energy applied to the open end of the tube, its length and probably other things. As you go faster, the air hits the end faster and when it gets to the right speed, the sound begins. I don't know for sure, but possibly putting a piece of duct tape or something over the end, or plugging it with a piece of plastic or a cork might work. You can try filling the tube with something like paper or plastic, too, if the first idea doesn't work. The shorter or smaller the vibrating thing, the higher the frequency or pitch. Since women are on average smaller than men, and on average have smaller vocal cords, their voices, on average, sound higher because the smaller vocal cords vibrate at a higher frequency.