I wouldn't at all be surprised if it were the mice in the walls!!! I had a Jack Russell who "told" us we had something in the attic (turned out to be a squirrel) - he would stand under the noise, jump up and bark. I guarentee you that if he were closed in a room and he heard that noise in the wall, he would have chewed COMPLETELY through the wall
I have no doubt. I bet that is exactly what is going on!!!