Chances are, there’s a small divot cut into your idler wheel. As the wheel goes around, it makes a knocking noise every time the divot hits the drive shaft. This is fairly easy to fix. If you remove the idler wheel (that’s the black, hard rubber one) and fill the divot with super glue, you can later sand it down to be equal with the edge of the rest of the wheel. I’ve had good luck with this process. If you don’t trust yourself to do the job, you can always simply have the idler resurfaced (see above), which probably needs to be done anyway.