It seems like every evening/night, I hear a strange low-frequency vibration that seems to come from the apartment above me. It's not a rumble like someone watching a movie - it's a steady vibration like a large appliance or a massage chair or... ...perhaps a Sybian . :) The vibration even changes modes, like there are really two vibrations of different frequencies and you can hear the pulses. Then suddenly it stops altogether. But I don't hear any screaming or other such gratifying sounds. Hmmmmmmmm..