To test for a spun rod bearing, remove the spark plug wire from one of the rear cylinders and start the engine...
If the noise is the same, shut down the engine, replace that spark plug wire and remove the other side rear spark plug wire and restart the engine...
The noise will not be as violent or harsh when the cylinder affected is not firing.
If the noise drops in level...(it wont totally disappear)...you have a spun rod bearing and the engine will need to come out...
Most likely scenario was the original failure of the fuel pump diaphragm allowed fuel to enter the oil pan, diluting the oil and causing the rod bearing failure.
