1 - Best option is to get it to a vet
2 - Check the nasal area for dry skin that may be causing a whistling sound as the air is evacuated from the snakes nostrils If there is dry skin on the nostrils, try gently to remove it. If it won't come out, give the snake a little soak in luke warm water for about 10mins.
3 - If there is no dry skin on or around the nostrils it sounds like a possible case of Resperatory Infection (RI). Watch the snake breathe very closely and check for bubbles or "wet" looking nostrils. If you observe either of these symptoms, keep the temps at a steady 35 degrees with no night time lows. Reduce the size of the water bowl to lower humidity. Take it to a vet at the first possible chance.
Again the above is only what I would do if I couldn't get it to a vet. Considering that vets are usually pretty cheap where reptiles are concerned there are no excuses really. It's impossible for us to diagnose the condition over a forum on the internet. Your best option is to get it to a professional.
Cheers,
Peter.