The soft noise
hear inside my head
when I listen for it, or when it is really quiet, or late, or I'm really sick, even if I don't try to listen for it.
The sound is most basically a soft, harsh, high-pitched tone. But it is not perfectly static. It has a soft of subtle vibrato. There are all kinds of overtones and under-pitches to it. The volume varies, depending on a number of environmental factors, health, stress, etc. I can tell when I am getting sick by the quality of this sound.
I like to think that I am hearing the impulses of a trillion neurons, a sort of pulse or the background noise of my brain.