Because everyone says it belongs to the overboard exhaust valve it doesn't make it right.
The pencil sized drain you describe is incapable of producing the sound due to its dimensions. The rush of air heard is of large quantity and would't be possibly to flow through such a small tube and make such noise without additional physical signs. Although you are correct in that it is fitted it and its use it is not the source of the sound.
On occasion a pilot can observe a droplet of water in said tubes, but this doesn't move when the sound is occurring during descent, meaning at that time there is no air rushing though the tube as you described as a possibility.
I'm sure the boys in Boeing explained it properly in our Type Rating course, many years ago, when a pilot asked that same question having been on an observation flight prior to conversion training. If not, ...