Because she is a new cat, you still need to learn her ways and she needs to learn yours. She is probably telling you to leave her alone or to stop what you are doing or whatever the situation may be. She may also still be learning to trust you. So if you do something, even by accident, that crosses the boundaries, she will hiss to tell you to back off. Just take good care of her, and treat her kindly and be very patient and you will see what causes the hissing and soon it will stop.
How old is she BTW?