It's called tinnitus, and is pretty common. Nearly everyone, when it is quiet, will be able to hear a high-pitched buzzing which is the result of hearing damage. (We all have hearing damage unless you like, grew up in the woods, everyday life around us is too loud to not cause small amounts of it.)
Your description of it getting louder is more unusual. Could easily be an ASD thing, working in the same way that some people will find everyday sounds too loud at some point and not others. And this is a sound like any other sound for those purposes. Could also mean you just have tinnitus worse than other people. People who have it strongly will nearly always notice it more when it's quiet, and for many it does not exist when there's something to focus on. Plainly, the lack of focus of something else is making you focus on it, and that coupled with the ASD thing where a sense's salience is not well-controlled, it becomes out of hand quite easily.
Older people are the ones who tend to report more serious issues with tinnitus, and also weirdness, like instead of it being buzzing they might hear music, or the same song, over and over but like it's actually there like tinnitus, and not just in their mind's ear, and it will go away if there's other music on.
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Not autistic, I think
Prone to depression
Have celiac disease
Poor motivation