The rod knock in an engine can occur when the bearing has been partially or completely destroyed. It’s usually due to oil starvation although bearing wear can happen naturally over hundreds of thousands of miles.
On the flip side (literally) is the Wrist Pin. It’s a hollow pin that holds the piston to the top of the connecting rod. When there’s wear on the wrist pin, a condition known as “Piston Slap” occurs – the piston is slightly loose and wobbles in the cylinder, making extra noise.
Keep in mind that rod knock and piston slap are both caused by incredibly small changes in tolerances. We’re not talking about a quarter of an inch – we’re talking in terms of thousandths of an inch! That seemingly minor gap allows for enough movement to cause rod bearing noise because metal parts can now bang against each other.