It's really better to keep some sort of hot spot at night. Something in the 82-85 degree range just in case the snakes need it. I've certainly had success breeding with turning the heat off completely at night and using room temps, but since this is your first time breeding and you're not real familiar with recognizing and treating URI, keeping a slight hot spot is probably a better option. There's a strong chance that the whistle is just from an upcoming shed, but there's also a chance that it's not.

-adam