I'm not sure, sounds like a mix of hiss & fuss. What is she doing exactly when making this noise? I've read that noises while trying to pee is a good sign medical attention is needed. Noises while pooing can be just constipation, but needs to be watched to see if it improves. Other than that, I believe it's just noise, communicating something.
Sorry I can't be more helpful. Like I said, if she's not going potty, I don't think it's anything to worry about.