Mine were scheduled for 7:15am. I dragged feet into the clinic because I didn't get much sleep the night before wondering how things would turn out.
MRI specialist told me to lay on the bed and relax, so I did, and took a catnap for about an hour. I couldn't hear the loud noise it made, but I could feel the vibrations.
It sounds like you're inside of a heavy iron barrel and some fool is pounding on the outside with a lead-shot hammer....and then the tone/rhythm changes *just* as you were getting used to it...;))