First of all, don't panic. You are experiencing Sleep Paralysis.
I've been living with SP since I was about 12 years old, it's frightening, frustrating and indeed very bothersome because it robs you of sleep (personally, I can't sleep after a night of on-going episodes).
There are numerous 'explanations' for SP. Some people say that it's 'spiritual', while the scientific community has a perfectly good explanation for it as well. There are even those that believe that we're dealing with Alien Abduction here (read somewhere years ago that the buzzing sound is actually due to the astral body being pulled into another dimension where it is experimented on -hence the physical pain you may feel sometimes).
Personally, I have made peace with my SP. While I do accept the medical explanation, I am also open to a 'spiritual' explanation because I've just seen too much, dreamed too many things and experienced lucid moments especially during sleep-wake phases which, medically speaking, I cannot explain.
I am now 36, they say SP is supposed to go away with time but mine has only worsened. I now experience intense audio hallucinations, lots more sleep-wake episodes, I have a difficult time breathing and I even feel more pain than before (around my legs or abdomen). I used to fight it and I found that when I fight it, it frightened me more so by making 'peace' with it, I discovered that it's easier to pull myself out of it.
Also in my personal experience with SP, I have explored things like Out of Body Experience, it's kind of scary at first, but again, I've learned to embrace the fear. Now, I just enjoy the ride. I pay closer attention to the voices but I usually forget - completely - what was said to me, no matter how hard I try to recall. Whenever I feel that sensation of a "presence", I ask questions like "Who are you" "what do you want" or "do you have something to tell me". Sometimes I get an answer, sometimes I don't, sometimes, I feel physical pain when I ask too many questions.
I've come to realize that this is, on some weird Freudian level, my own subconscious mind... but then again, what are demons and angels but our own subconscious, right? Right.
As far as the medical explanation is concerned, I also believe there are triggers. Something I'm doing is causing this 'CHEMICAL' reaction in my brain, so I've analyzed this as well. I've noted that I experience SP when I drink a lot of coffee, when I am under a lot of stress or when I go to bed angry or emotional (sad, depressed, etc). Now that I have overcome my fear of SP, I find that I sometimes WANT to have SP! Sounds weird, I know, but it's kind of like a roller coaster ride... it scares you but you want to do it anyway.
Some people say you shouldn't sleep on your back, but it happens to me when I sleep on my belly, my side, it doesn't really matter. I find that I can pull myself out of it by relaxing, then wiggling my toes the minute I hear the buzzing sound approach. If you wait until after the buzzing sound has 'consumed your head', it will be more difficult to pull yourself out.
Don't fight the paralysis, that's the worse thing you can do... just ride it out, learn from it. Nobody has ever died from it.
Waterloo University did a great study on SP.. I'm sure if you Google this, you will find tons of info on it.