~~~I caution people here as to deliberately try to catch yourself falling asleep can lead you into deep water and you might not be very employable if you venture very far; though that is not necessarly undesirable.~~~
I've actualy done it quite a few times, although not intentionaly.
What happens is this: I find myself in bed going over the days events. I'm stressed out about car payments, impending transmission problems (so far they're minor but we all know how car problems progress), thinking about my girlfriend, what I want to do with my life ... you know, all the stuff you think about before sleep takes over.
I'll be on a coherent thought, i.e., "I need to schedule a dentist appointment tomorrow," when all of the sudden my thought will derail and turn into nonsense, "I think I'll make the appointment for one o'clock because ... thirty two o'clock because my dog's transmission is going to screw up my girlfriend's life."
I crack a huge smile and sort of chuckle, "Where the heck did that come from?" I'll think to myself. It's as if I lost control of my conscious mind for a brief second. I hate to admit it, but that's happened to me before while driving ... pretty dangerous!
Or how about this one: I've nodded off watching TV when all of the sudden a baseball flies out of the screen and slams into my face. My whole body jerks as if I was overtaken by a violent convulsion. I awake only to discover that the TV is just fine (no advanced technology here) and there is no baseball in my living room. I happened to simply make a smooth transition from awake to dreaming while still watching TV. The exact moment of sleep was when the baseball left the screen! Pretty cool I must say.
It's like I briefly lose control of my brain, and then nonsense takes over. One time I was even attentive enough to place my "drifting off" between two consecutive words in my self dialouge (when I think to myself), therefore the "moment of sleep" was pinpointed between the last word that made any sense and the nonsense. I, in essence, caught myself falling asleep.
That baseball dream comes in many different forms, it could be a basketball, or flying bird, or my ceiling fan can fall on me. And it doesn't happen to frequently, but it once may have saved my life because as I mentioned ealier I was driving when a flying bird hit me. I jumped up and noticed I was changing lanes (while dozing off) but luckily there was no traffic (it was about 3:00 am and across town from the LV strip; no tourists...), so I simply let my car go across the line into the next lane as if I had meant to change lanes all along. Then I got off on the next exit and went to a gas station to get some sweets and something to drink (so I could drive more attentively).
~~~What I'm thinking is: having got practiced at noticing what goes on at the sleep-wake boundary; I was perhaps used to having a corner of lucidity during unusual consciousness states.~~~
There was one time that I can recall where I noticed the "nonsense thoughts" overtaking my normal thinking. I said to myself, "hey I must have just started falling asleep," but then I noticed it was daytime out. "Hey wait a second," I thought, "just a second ago it was 11:00 at night." I noticed I was no longer at my house but at my favorite cousin's house. "Hey, I must still be asleep!" I got to experience what it must be like to be God, because I was in total control of this surreal universe. Since I knew I was dreaming, whatever I could imagine, I could bring into existence right there on the spot. I could bring people back from the dead, fly, create and destroy things with my mind ... I could do litteraly anything! It was a really cool dream except that at some point I must have lost control of it; it turned into a "normal dream".
~~~When I listen to "the transparent background sound" in quiet surroundings; there seems to be a second, lower pitch, more bell-like sound. I think quite likely that aspect of the background sound covers experiences from before birth.~~~
Do you live in a city? Perhaps this can be explained by the fact that you may be more sensitive to noise (generated by machinery or traffic or something off in the distance) more than other people may be?