I have tinnitus. It manifests as a sharp sinewave as well. For a week however, the frequency of the sinewave dropped. It sounded exactly how a medium frequency sinewave from my phone, at around maybe 500 mhz( actually its hz) , and it could go down to around 400. It was just for that week, and then it stopped.

Your comment reminded me of that, and I thought it’s an interesting enough story