I have noticed more than once a sound, kind of like rumbling, or maybe like how I remember pigeons sometimes sound in their nests, coming from the box. I have listened to sounds of queens piping, and this is much lower. I went through the box and didn't find any mice, birds, or what not. Never the sound on the frame I'm holding. Has anyone else heard anything like this? I had suspected this box being queenless, but did see eggs on the bottom of the cells. Some had more than one egg. But there was capped worker brood. Not that that would relate to the sound.