My brain is never quiet, either, both in the "constant thoughts" sense and the "constant noise" sense (like a low-level hum).
I've had several hearing assessments recently and thought the hum was related to a problem with my ears...maybe not.
@EstherJ: thanks for the article. "Low signal-to-noise ratio" is exactly what the audiologist said regarding my hearing results. She didn't think it was a brain issue, BUT the next audiologist I saw (in conjunction with an ENT appointment) said it very well could be (something like CAPD).
Trying to figure out what to examine next...this has given me something else to consider. 
_________________
The existence of the leader who is wise
is barely known to those he leads.
He acts without unnecessary speech,
so that the people say,
'It happened of its own accord.' -Tao Te Ching, Verse 17