Originally Posted by Moto
Thanks so much for the great info! What exactly does it sound like?
Unfortunately the exact sound varies with the severity of the knock, engine RPM, engine design and other factors. They can also be called a silent killer of engines because you can get just to the point where the knock cannot be heard but it destroys the engine. We only hear it when it is severe and unfortunately, the smaller engines it is just about impossible to hear until it is too late. The best recommendation I can make is pay carefully attention to your spark plug for aluminum deposits and look into the bore for signs of piston damage.
Oh as for the sound, it can sound like the metal end of a pocket screwdriver tapping on a hard surface and gets louder from there.