From my research on tinnitus, I have not found anyone that has explained definitively what causes the ringing noise. There are many possible ideas but no certainty.
Now, what is the cause for it starting from the first place. Again, mixed ideas. What I have read is it can be caused by
Exposure to loud noises either acute or sustained which leads to damage to the inner ear ‘hair cells’ or stereocilia. This then causes a lack of stimulation often in the high frequency range and loss of hearing.
Certain medications have been reported to cause tinnitus.
Impacts to the head and whiplash have also been suggested on the other websites.
What caused mine is unclear to me. It was accompanied by acute hearing loss. I often had trouble hearing in crowds but one day normal conversational hearing started to be affected.
I had back pain issues with accompanying sciatica which I treated with OTC analgesics and stretching. It had resolved although I have had issues for a long time so it is always there. A few weeks later, I exacerbated my back issue and the sciatica returned which is when I noticed the hearing loss and the tinnitus started.
I resolved the back pain but the hearing loss and back pain continued.
I went to see an otolaryngologist and he told me it was due to hearing loss which was due to my exposure to loud noises when I was younger and it reached a point where it became noticeable and resulted in tinnitus as well. He did not think it was due to the other reasons I mentioned.
The only treatment he offered me was hearing aids. No further investigation or study. No trying to understand the root cause. I did have a head scan to make sure I did not have a tumor on the eighth nerve. I read a tumor on the eighth nerve would cause issues in one ear more often than both. In any case, my scan came up ok.
So I do not know for certain what caused this all to start. I also do not know for certain what is generating the noise I hear. I am sure you may be frustrated by this as well.
Nonetheless, there was a cause that started this and there is a cause for the ringing. The latter is my focus although thinking about what caused it to start is also helpful.
I am not a healthcare professional. Please seek professional medical advice for your medical issues and care.