My own noises are Normal and Fine

Everyone else's noises are Evil and Mean

One way I've seen this explained is that the bad sensory stuff is things that you can't control. Whereas the humming or chatting or music or whatever is something that you're in complete control of. It's noise that you want and expect, not noise that's disrupting you.

your noises have been processed by your brain before you even made them, so they're not bad

I have misophonia and sounds that I make myself are definitely included. I try to minimise the noise I make at basically all times.

Solutions exist.

My own noises definitely bug me sometimes, and I generally try and keep quiet. Can't really recommend, though. It definitely contributes to making one's self an uninteresting wallflower.