It is very common for snakes to whistle when in shed. Your temperatures and humidity seem good as well, but a thermostat is definitely in order. I'm not saying there's no way it's an Upper Respiratory Infection, but with the information given I'd have to say it's just skin tightening her nasal passages as a result of going into shed that's making her whistle. Good luck, and I hope she's fine!
Cheers,
-Matt
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