Mine sometimes hiss during feeding - I've just taken it as a sign that they're getting excited about dinner.
Sometimes it can be gas escaping from the mouse as it's constricted.
Generally, hissing is an indication that the snake is getting worked up about something - could be agitation, warning or excitement.
They can seem to make noises as they breathe when they're coming up to shed, as the skin first loosens around the mouth and nostrils. This can cause irritation and sometimes causes a reaction that people describe as "sneezing". I've not seen it in any of mine but it doesn't seem to be an indication of a problem.
If you can hear her breathe when she's at rest (and not due to shed), and/or you see any fluid or bubbles around her mouth or nose, then she could have a respiratory problem that would be worth a vet visit.
But I'd say hissing or sneezing in isolation is probably not anything unusual. Just keep an eye on her for any other trouble - chances are you won't see anything, but it never hurts to be vigilent if you're concerned.