The Sound Above Room 12

Early on into their first semester, Oliver noticed a strange sound that seemed to be coming from the roof above his dorm room; it is a rhythmic thump, almost like a metronome, that occurs at the same time every Saturday night. After Lucretia and Vice found out about the sound, they investigated the roof but didn't find anything. Later while investigating the babau incident, our protagonists discovered a stairway leading from the staff rooms to the roof-- that passage could be the source of the sound, but the position of the trap door and stairway is far off from Oliver's room. At present, the source of this sound is still a mystery...