Well, I would stick with a description of the sound rather than trying to turn it into a collection of letters. Every reader would pronounce it differently and it actually would make me stop reading, lose the tension in the story, and physically try to say the word out loud. You could try something like "a curious startled snort, the warm/ freezing air  exhaling/ inhaling with a soft woosh." What would I know! Good luck with your spelling and grammer checks. Just send it along to me and I'll proof read it. Ha, ha.