Note: This question has nothing to do with pressure change.
When I'm flying in an aircraft at cruising altitude, the monotonous sound often lulls me to sleep. I've noticed that just as I am on the verge of drifting off, the perceived sound volume drops significantly. I then notice what is going on and the thought awakens me a bit and the volume snaps back to normal. I verified that the actual noise in the airplane stays constant while this happens.
I always try to hold on to the state where the sound is quieter, but it is very difficult to do while at least a bit conscious. That's why I suspect that it could be part of the process human brain goes through normally when falling asleep. Perhaps it happens every time, but I do not notice because I only ever sleep in such a noisy environment when flying.
I asked a bunch of friends and some of them experience the same phenomenon. I was unable to find any relevant scientific literature mentioning it, though.
What causes this fascinating phenomenon? Does it have a name? Has it been studied?