This is straight-up tinnitus. Ever since I had ear surgery, I have various types of tinnitus. When the low form started, it reminded me of an idling diesel engine. But of course I mainly get the kind that is easy to mistake for cicadas. Sometimes I've had static sounds, too.

When I was still healing, I would also get phantom choral voices! That was really disturbing. It usually was triggered and filtered by the bathroom fan, so I basically got to crap in a heavenly soundscape, lol.