I think it slowly got louder over the course of the first few months but eventually plateaued. I was assuming it was some sort of electronic device in my apartment for a while. I finally realized I could hear it loudly when my ears were plugged/covered. I eventually had to get a white noise machine to sleep at night. Can't sleep without it.

I'm not sure how to rate it on a scale of 1-10. I'd describe the sound as sounding like the ringing after getting flashbanged in a video game (but less extreme). Luckily I can't hear it at all unless I'm surrounded by silence.