I want to throw in another thing to test. If you only hear it in one spot and you’ve checked everything else go buy a carbon monoxide detector and take it where you sit. It can cause hallucinations.

If you’re hearing something you need to know the range you can hear and this gives an idea of frequency. As someone said above unplug and reboot everything. https://youtu.be/-E1SDl9vLo8

Edit: there’s a 30000 hz tone teens can hear. Now I’m going to throw in there’s a sleep thing where you might here noises that aren’t there right when dozing off and waking up. I mention because it’s common. It’s annoying. A lot of people have it. Here’s the name. It’s not a mental disorder. Stick Earplugs in and see if you still hear it. hypnogogic hallucinations

Also grab a different set of headphones.

I just spent months looking. Found something that would work. You did amazing! Great job!

It won’t make clothes smell I used to use as fabric softener. No smell. I’ve also used vinegar and baking soda to strip chemicals off sheets and make them soft.