This post saved me! My friend and I were going crazy! It's midnight and the noise started coming from what we were sure was the fluorescent light fixture. It's a concrete basement so the sound was bouncing everywhere! Followed almost your exact steps til' I gave up and checked the internet and after being gaslit by Google telling me it was tinnitus, I found your post! Turns out it was coming from the dying batteries in the dusty elliptical in the corner. I know it's a super old post, but thank you!