When I was growing up, I could hear something inside the chimney that went up through my bedroom. No one believed me until my father heard it too.
Send help!
Also, get than animal checked out. It might just be a mouse -- those things sound like a freaking saccoon sometimes, they're so loud -- and nothing much to worry about, but if it's anything bigger, then it should be dealt with immediately.
I was upset when I got home from school one day and there was a hole in the chimney and the window was open. My father broke into the chimney and pulled out a black duck (normally white) from the chimney. I didn't get the whole story but I know that the window got opened and the duck flew out heading for the pond.