Do you notice her opening her mouth or gaping? I know big snakes have a tendency to breathe harder/louder, but usually only if they are active around their enclosure I would think. I would definately check her mouth for bubbles or cheesey looking stuff. Also I would check her nostrils for left over shed. Sometimes when their nostrils are blocked it makes that whistling noise, and if you hear it all the time, that could be what it is.

Good luck, hopefully it isn't anything too bad.
Jennifer