What is under my house? and how do i keep it away?

I live in a mobile home park. Our home just so happens to be next to a field. At about 11pm every night I hear something banging around under my house. Im not too sure where exactly the animal is (in the vents or under them) but when they bang around, you can hear it clearly through the vent in the computer room, (coincidentally the room I'm in right now) and they are banging so hard it shakes the floor where me feet are. I'm thinking it's a possum, but I've done some reading and they are nocturnal...wouldn't they be outside right now? Anyway, the noise is kinda scary because it just happens out of nowhere and I'm afraid they might get through. Also, in our hallway, is the thing where the main controls (I guess you would call it) for our heating and a/c. I also hear them hitting that pretty hard and it sounds like they are in that area. This is a concern because the wall it is next to isn't made of brick, if you catch my drift. Sorry it's so long but I think it's a possum or a cat. One scares me the other doesn't. Another reason I think it's a possum is because the night my husband brought this to my attention it started hissing at him. But it wasn't a cat hiss, it was more of a heavy breathing hiss.