If your Toro self-propelled lawn mower starts and runs fine but then makes a knocking noise while you cut the grass, check the blade. The blade can become loose which just needs tightening or something may be stuck like grass, a rock, or other debris. If that still does not stop the knocking noise then you may have a bent blade or it may be out of balance. If the blade is bent, then it would need to be replaced.
You can find the model and serial numbers located on your Toro push or self-propelled lawnmower on the top back area of the frame.
NOTE: if you need a part for the small engine in your Toro push or self-propelled lawnmower, there will be a separate model/serial number for the engine. Here are the small engines that are available in your Toro push or self-propelled mower. Visit the page of the engine you have to find out where the model/serial number is located for your small engine model: