Cody you did a good job with the answer!
Typically a whistle can be caused by a few things, a respiratory infection, retained shed, or the snake is getting ready to shed.
If the snake has not shed recently or has not recently had it's eye go cloudy and then clear back up you can rule those two possiblilties out.
As for the resp. infection.... they are usually caused by improper cage conditions such as too much humidity or temperatures being too cool. If you do not see any crusty stuff around the nose and mouth or bubbles coming from the snakes nostrils you may be able to treat the issue. Get a digital thermometer and measure the temps in the cage. To treat the RI you want it to be 90-91° in the entire cage. No cool side! Watch the humidity levels as well, you do not want the humidity levels to go above 50% or it could cause problems for the snake. Keep a close on eye on the snake for the next day or two and see if it gets better. If it doesnt get better or shows any signs of worsening get to a vet immediately! You may want to schedule a vet appointment even if you try and treat the RI yourself, just to be safe.