I have had that"pile driving" sound in my head once and it certainly was different.
The only thing that made any sense to me was that I was breaking through to another dimension, mentally.......
No, Dear, you are not going crazy. When I am just drifting off into sleep, I hear people talking and that's when I know that I'll drift off and wake up when it's morning.
One night I heard a man say, "Honey, where are my socks?" so I guess that I was picking up the conversation, telepathically.
When I sleep and have vivid dreams, gosh, it's like I'm in conversation for hours.
And, yes, it could be that you are on the Astral Plane and either those voices are, Spirits or other people from the Earth on the Astral.
I believe that these things are real and true.......