The drive belt is the usual culprit. If you can stick your phone down there and take a picture of the sprocket that isn't covered by the belt then do it. If it is worn and shiny and there is black dust everywhere then you have found the problem. Once the belt blows by the hard anodized surface then it hits the softer aluminum and eats it like butter.
If it still looks OK then adjust the tension. I have found that all it takes is hitting a pothole at speed to mess it up.
