It's the belt. Check for tension. Look for wear. If there is wear, as in, the teeth of the sprocket are wearing down and are very shiny and if you can see that the belt is wearing a 'step' compared to where it doesn't hit then you definitely know what is going on.
To me, this is only the beginning. Once it is doing that then it is only a matter of time until the noise will drive you nuts and the degradation will get worse. When the sprocket is new it ships out with a very hard anodized plating. This is only a few thousandths of an inch thick and is very hard but it does wear and when that is breached then the belt is into the soft aluminum which it will eat like butter.
Aside from having a stone go through it the belt is very tough and should last 100,000 miles. At this point the belt is far tougher than the sprocket.
Search this forum on this subject. Much in there. I'm in most of them. Sore subject...
"Tighten the screws on the back of the seats & around that area" is a cosmetic hit only. When you do that and you remove the seat bottoms and see a lot of black dust then that is a clue. That is pure aluminum from the sprocket.
Tightening the belt will make it wear faster but will alleviate the noise for a bit. The belt should not make noise at all, loose or tight. It's a materials issue not a tension issue.
