Jeff,

Excessive wear on the thrust plate will cause a knocking sound.

When you torque the timing gear nut tight, the spacer on the nose of the cam get clamped tightly between the back of the gear and the front of the first cam bearing journal. If you didn't have the spacer, the thrust plate would get clamped instead.

The spacer is supposed to be .003" thicker than the thrust plate, so if the thrust plate has no wear and is completely flat on BOTH sides, the cam can only move forward or backwards .003". The back of the gear and the front of the journal DO contact the thrust plate and eventually it wears down on one or both sides. When that happens, the difference in thickness becomes greater than .003", and the cam moves forward and backwards more than .003". As the wear increases and the cam moves further, you'll start to hear it banging into the plate.

The spacer does not touch any wear surfaces and normally lasts forever. The cam and gear hub are a lot harder than the plate, so it's only the plate that wears significantly.

Of course, it could be that you're having another gear failure, but if you didn't replace the plate at the same time as the gear, I'd sure pull it off and look at it closely.