The scariest thing I've experienced has been sleep paralysis.
Never had that, but it does sound scary.
http://en.wikipedia.org/wiki/Sleep_paralysisWhen I experience this, I begin to hear a high-pitched ringing noise (or occasionally a dull roar) that gets louder and louder.
I often have an icy chill on my back and a feeling of dread, as if I MUST stop the noise before something bad happens.
The last time it happened, the roar was accompanied by a very brief dream in which I was in a car driven by my brother,
and we were going faster and faster down the highway as the roar grew louder.
Fortunately all I have to do is twitch my leg or something, and I snap out of it.