The self propel feature on my Toro 20066 self pace mower is making a lot of noise. It is acting as though the gears won't quite engage. It has a vibration and grinding sound when it is engaged. I can loosen the cable to make it stop and then it disengages the self propel feature. I've changed the traction cable, but it continues with the same grinding and vibration even after adjusting. What would be the next step, belt, transmission, ??/