It is possible that stuck shed is built up on the nostrils causing this. RI has some tell tale signs outside of noises and whistling may be a sign of shed either oncoming or stuck. If there is no other signs of a RI then the best thing to do is to make sure humidity is set around shed time and that they have a complete shed next cycle.

If you do feel it getting worse and see visual signs of mouth infections (heavy saliva, slack jaw or discolored discharge from the corners of the mouth) or see that the snake is constantly holding the head upright and seems to act like they are resetting their jaw outside of feeding then you can take the next step and find a qualified vet. I hate to see someone make an unnecessary trip to the vet so for now just observe for signs outside of whistling which may just be shed related.

If you do see any of the symptoms I pointed out above especially slack jaw and discharge from the mouth, it is best to let a vet evaluate the snake and not try and treat this yourself.