If it's a cat growling, then it's really scared and angry, and possibly feral. A hissing cat is about to fight its way out, in my opinion.

It may be a mama who gave birth under your stairs. If you keep feeding it, you might have lots of little kitties on your hands.

No, not WAR kittens! http://www.youtube.com/watch?v=yosTIwbFjzM#

I think the best thing to do is get the animal control folks and get it out of there. If it's feral, you don't want it setting up residence under there. If it's not feral, it probably has a home and someone's looking for it.

Karen