I've been in row 13 (last row window) on American's 777 several times. I like that corner of the business class cabin, because on night flights it seems the quietest and easiest for sleep. What I notice, though, is a periodic low sound accompanied by a very slight vibration from somewhere in the aircraft. It appears to go on and off about every thirty seconds. It is not toilets being flushed. This row is over the wing on these planes.
What might that be? Would it be fuel pumps?