Everytime I have what I guess is hypnagogic sounds, they’re like computer sounds, or sounds that robots make. I once had an SP and I noticed I lied a bit to my side and when I tried to move my body there where this buzzing noises right over my head. And once I was very sleepy and almost falling asleep and everytime that almost happened I heard some noise like a machine is being shut down. You know? This always freaks me out. Is this common?
Does not sound like the same sounds that i have. What i hear as i slip into SP is something i can only describe as wind blowing in/by my ears and some rare cases a high pitched sound that varies in frequensies.
Other sounds that i hear can be anything between voices, knocks, laughing or even music.
These sounds are as vivid as actually hearing them with my physical ears.
I think everybodies hypagogic sounds are different. I experience mine usually as falling noises of different things (like a bottle of glass on metal etc), seems to depend on your surroundings.
Mine is organ music. Everybodies is different, lol.
Mine are usually random sounds and especially voices, sometimes music. I’ve actually never heard any “robotic” hypnagogic sounds… which means I agree, I think everybodies hypoagogic sounds are different.
I’ve heard extremely exaggerated noises coming from IRL (white noise) and voices.
I’ve had train noises, wind, banging, scratching on glass, and bouncing of a basketball. I also hear them as vividly as I would IRL. Once I laughed because I heard what sounded like a fight between two cats lol. 
When I had the SP mentioned in the first post, I also heard music, but with parts of the music taken out. It was a song my brother is working on a few rooms next on the computer, he sometimes lets out frequencies to make the guitars clearer. So the “taken out” parts made me think my brother was there and gave me the feeling of a presence, pretty cool. I also thought my mother was in the room in front of me, there is a see through door. None of them was there.