I will tell you exactly what it is. There is a rubber stop on your lower cam. When you draw your bow, just before the cam rolls over your string will rub against the stop makeing a rubber band doink. Push the stop with your finger in toward your cam enough that your string no longer hits the stop. Trim just enough off if you can't push it in enough. This happened to me also and drove me crazy until I figured it out. Pushed it in and no problem.