Strange Buzzing noise when im just about to fall asleep?

Please read, its not Tinnitus. A few times (about 5 times the last few mounths) when ive been at the point when im just about to fall asleep, i will suddnly get this electrical buzzing sound that will get very loud very quickly, louder as im about to fall asleep, it will last about 3-4 second, untill it gets so loud i bolt upright and compleatly wake up. It will start out as a very distant beep, then as i ignore it and about to slip into a long sleep (two seconds later) it will get extreamly loud very quickly and jolt me upright. Its only happened a few times, and the buzzing only lasts for 3-4 seconds and it ONLY happs when im just about to fall asleep, its not Tinnitus, im almost certain its got something to do with the brain falling asleep or something. What do you think it could be?