You did the right thing going.
If the doctor ruled out known physical causes, there's not much we know about possible causes.
In many cases it will go away on it's own, but the longer it lasts, the more likely it is to stay (become chronic).
Try to minimize stress and try not to freak out about it. The more you can ignore it, the better. Listening to quiet music, podcasts or white noise helps not focusing on it when going to sleep.
Look at potential bigger stress factors, too: My doctor once told me about a case where the tinnitus disappeared after a engagement was cancelled.
You can also have your neck/shoulders and jaw looked at. Tension there might be causing it. Massages help with relaxing, too.
If it does end up being chronic, relaxation is still key. Music notch therapy shows also promise.
But it's too early to worry about that. My wife just had a scare, and it went away after a week.
Mine is chronic, but I (literally) learnt to live with it.