Whistling can be a sign of a respiratory infection. Especially if it's every time she inhales. How long has she been doing this?
Any kind of consistent whistling or wheezing isn't normal. Can you describe your enclosure and temps/humidity? It may help find the problem if it is environmental.
The basking area should be around 90 deg F, so you might want to crank the heat up some. Sometimes too cool temps can cause respiratory problems in reptiles besides the obvious infections. Just make sure there is a hide box for him to get away from the heat.
you need a humidity of 70% for redfoots there nasal passages become dry in low humid enviroments causeing whistling noses as they breath they also require a water dish large enough to wallow in