Smaller water area, more holes in tub (done slowly so you don't over do it), moving the water further from heat source (if possible) are all ways to lower and better mantain your humidity. As for the whistle, need more info on your temperatures, such as Ambient temps, hot spot temps, and cool side temps. . . Whistles can be the start of an RI, be very watchful for any kind of dried up or wet discharge on the snakes snout.