I have heard to raise the temps and to increase humidity. Increased humidity will help loosen up mucus.

I think I remember seeing a recent post from you about your snake being about to shed. Has he shed yet? Sometimes as the skin is starting to loosen a loose piece in their nose can make whistling sounds when they breathe. Even if he has shed, if that is when the noise started, I would say it is possible that a little piece got left behind.

Definitely watch him very closely, and if it gets worse, go to the vet.