The whistling noise can be from a piece of leftover shed skin (or loose skin from an impending shed) hanging weirdly in his nostril. I wouldn't worry too much unless he starts making a sort of wet gurgling or hissing/clicking sounds. If he's still making this sound after his next shed, you might want to think about taking him to a vet. These things are easier to handle if they're caught early.
Pick him up in a very quiet place and put your ear near his body. Do you hear any soft clicking/snapping coming from his body? That's a clear indicator of an RI.
It's getting close to winter, and a lot of snakes do come down with RIs around this time. It probably wouldn't hurt to bump up his ambient temp just a little bit, to err on the side of caution.
Edit: Do you know what the temperature is in the cool hide he seems to prefer? Are the hides all different from one another? He might just like that hide in particular and isn't choosing it based on the temperature. It might be too cold for him there. Maybe you could try moving his favorite hide to the warm spot and see what he does.
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