Is it possible that what you are hearing is a very low hiss? One time a new snake I had just gotten was doing that, and since the other snakes I had had up until that point either didn't hiss, or hissed comparatively loudly, it didn't occur to me at first it was a hiss, and I thought RI.

When you say she has had this "for some time", how long is that? Another possible reason for sounds when they are breathing is a bit of loose shed in their nostril, but that should only occur while they are in shed, so I would think that is not the case here.

It is a good sign that she is eating, but if it is an RI, you should get her to the vet before her health goes further downhill. Don't wait for it to get so bad she stops eating, has bubbles in her mouth, etc.