I hear it also. The Kauai Monastery's writings say that the high pitched "eeee" sound is caused by the Ida and pineda nerve currents.
(I may have then spelled wrong) Each is at a different frequency. You should be able to hear each and the combined tones. (Of course this is not including any physical damage). Sometimes it is Very Loud.

Also, when you hear a tone, they say beings are known by their tone. But; I think this is when you hear a tone suddenly. This is not the same as the steady "eee" sound.

If it is not too bothersome,, concentrating on it can produce the trance state.

I'm only stating what I've read and experienced.

Grant