Sounds like tinnitus, which is a constant noise in the ear. "Crickets chirping" is a common sound; I've had it mildly on occasion (certain medications can have tinnitus as a side effect.)
Tinnitus can be caused by years of overexposure to high volumes of noise, illness or medications.
I think I have it mildly, but I'm so used to it I'm essentially "deaf" to it, in a way. That's how some people deal. It can vary in severity and intensity based on the amount of damage and the individual.
There's some great info on wikipedia at this link:
You can also just google it yourself for more info.
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