bob - these guys are exactly right. Wistling can be an RI or skin from a shed. One of my snakes also got sick one day. It didnt turn out to be an RI but he did have mucas in his mouth and nose. The vet gave me some drops to feed him and things cleared up pretty quick.
What causes the wistling or weezing is some sort of obstruction in its mouth. Some snakes will also wistle to some degree when you pick them up due to their air shifting around.
Keep an eye out for any mucas, keep the humidity low and your temps high. If things get worse take him to the vet. Better safe then sorry .
Good luck!