What's That Noise?

All night, either in the wall behind our bed, or under the floor, or maybe outside, there has been a weird noise. I can't really define it... a scraping, gnawing sound. Too loud for a mouse. I'm listening to it now, as a matter of fact. We have a lot of "critters" here: opossums, raccoons, a skunk, even an armadillo, so I figure it's one of them, but what the heck is it doing? Of course, I've seen far too many bad sci-fi movies, so my imagination tells me it's...

Killer shrews!!!