Hi David,
I've never been able to find an answer either, and I've spoken with Dr Vernon, basically the founder of the American Tinnitus Association several times. Like you, sometimes I never think about it, sometimes it does get on my nerve, but that is getting more rare. Yesterday was a loud day, today it is very quiet. Go figure. I asked Dr Vernon that question, why the change during sleep, he said even they still don't know.
When you said white noise makes it worse, do you mean the loudness of it? Mine seems to want to compete with certain external noises. It's always been that way.