I think it's some type of mechanical equipment.
We stayed in 2635 once (across the hall) and every 4 to 6 hours we could hear a pump running or something. It only lasted for about 5-10 seconds each time and wouldn't have been so noticeable if the rooms weren't so quite. I still think the most noise in most cabins comes from the hallway and deck 2 aft has very little traffic and is a great location.
Bruce