Did it seem almost real? sounds like Sleep Paralysis. I had it bad twice before, one time I was just lying in bed facing the wall, and a hard, cold hand reached over my shoulder and dragged me down the bed. It was terrifying because it was life-like but I couldn't even move, or scream in response. For some reason, as I'm getting Sleep Paralysis, my Tensor typani muscles make a vibrating noise similar to the one you hear just before something bad happens in horrors. One time I had Sleep Paralysis while under my covers, so the rumbling sound was interpreted as ruffling above me, coupled with the fact I couldn't move led to a very strong hallucination of one of those crawlers from Black Ops 2 trying to tear through my sheets, I tried to scream, but it just made thing worse.