Hi and welcome. Sometimes medication can play tricks with your brain. If you are currently taking a perscription drug, consult your doctor. Another explaination is, are you experimenting with out-of-body techniques? This could account for the buzzing noise you hear. People who experience OBE's often report of experiencing this. It might also explain the strange beings you encounter before waking up.
Feeling that a strange being is pinning you down and making it impossible for you to move is a symptom commonly associated with sleep paralysis. Normally, when you are dreaming, your body becomes temporarily paralysed so you don't act out your dreams in waking life. (For example, if you are dreaming that you are in a fight, you don't want to be hitting the person who is in bed with you in waking life.) In sleep paralysis, your brain's timing is off, so you start to wake up before the paralysis wears off, so you can't move even though you are conscious of what is going on in waking life, but because you are awake but not 100% alert yet, you can have strange hallucinations. Many people who suffer from sleep paralysis report having a weird creature sitting on their chest. I guess that is part of the mind trying to think of a rational explanation for why they can't move.
I have never heard of sleep paralysis being dangerous in any way. As for the story about your relative's grandmother, I wouldn't rely too much on third-hand accounts of anything.