We have mice in the bathroom and kitchen ducts. I'm pretty sure about that. On and off I hear them running very fast with their little feet and making huge noise because the ducts are made of metal and have that echoing quality that makes a small sound ten times louder as if a roar.
Sometimes I miss them. For days I hear silence and I think they're dead. But then a quick sound of those small creatures ,as I imagine grabbing their cheese and running fast, makes me feel relief. They're back and they're alive.