I was curious about a number of strange experiences
that I experienced while I was in half-sleep state.

It is always the same, and it happened to me a few times and it is not very frequent.

Last time it happened it was in the middle of the night..
I was dreaming (don't remember what), and suddenly my heart starts pounding, and I start experiencing intense fear,
and my ear-drums start shaking creating a strange very low frequency effect.
Both the hear beats, the buzz in the ears and the fear
are getting stronger and stronger until I wake up rapidly as if I had a nightmare.

The strange thing is that the fear I experience is not caused by anything I dreamed or anything that I can think of, it is just as if my body starts producing hormones that cause fear for no reason.

The whole thing lasts for about few seconds.

And it is never a result of a bad dream or anything.
I'll describe one incident here:

i remember that one time I was dreaming that I was on my knees and I was lowering my head slowly to the ground as if I am falling to sleep. As my head was closer to the ground
the more my heart was beating, and the panic and fear grew, and the buzz in the ears was stronger.
One time it happened while I was falling to sleep (on the limit of being in the sleep state) and was not dreaming anything. That time I was sort of trapped in this ear buzz state and was trying to wake up but it was like a seizure.

The buzz in the ears, is more a shaky feeling than an actual sound, it is as if someone puts a finger in my ears and starts shaking fast. The feeling is not compleatly negative, it almost feels like scratching when you heave an ich (i hope this is the right word, my english is not very good)

but the whole experience is very negative, and the feelings
of panic and fear are very intense.
Sometimes when it happens in the middle of the night it leaves me very uncofortable even after it happened and I am awake.

What could this be? Did anyone have any similar experiences?