as mentioned whistling can occur from retained shed in the nostrils or heat pits. It can also be be a sign of a RI. You want to look into her mouth and see if you see any sign of fluid or mucos, that is a sure fire sign of a respitory infection. If your snake is not shedding in one piece it is a sign your humidity is too low and retained shed could be your problem.

Hope this helps you idenitfy the situation.