Morning Noises

It's 4:15 am Central Time right now. I went to bed a smidge after 12:30 am. As I was dozing off, I heard that familiar furniture-moving noise from the floor above. A few minutes ago, I heard it again. Now there's a perfectly good reason for me to be awake right now (my flight), but what on earth is going on up there? My friend suggested a few moments ago that maybe it was "one of those vacuum robot things." 

ROOMBA!

Of course...the vacuum-robot...Roomba...