[olddog]

If you over rev'ed the engine, it is likely that you stretched the rod bolts enough to allow the rod bearing to turn. When the rpm came down, the notch in the bearing was not aligned, as the bolts spring back to the normal position. This makes a tight spot on the crank and the bearing spins in the rod. Shortly after the spinning, the gap increases dramatically as metal is ground up. The large gap then causes the knocking sound.

If so, don't skimp on good rod bolts, when you put it back together.