It's early and I'm not coffeed up yet, so this is just some wild speculation of my own.
I can imagine if an excessive amount of oil, was making it's way into the combustion chamber,
That you'd be getting some carbon build up.
Now the the carbon will occupy some of the chamber space and thus raise the effective
compression ratio. That much I'm confidant of.
Now when the carbon gets pretty thick, it has been known to have some places
that catch enough of the (normal) combustion cycle heat for these places to begin igniting
your incoming fuel/air charge.
This would give you a knocking noise. depending on when it occurs, perhaps a little before the spark plug is fired (pre-igniton) or a bit later, say after the plug has lit the mix,
Then it can cause an extra pressure wave to race out fron it's hot spot and when that crashes into
the pressure wave from the normaly intended, spark igniton,
You can get such a sharp pressure rise from the two waves combined, that it hits the cylinder walls and head so hard that it sounds like something metallic is hitting them.
Either way that this second or unwanted ignition of the fuel is bad for the motor.
That clattering is as hard on things as whacking them with a hammer, if let happen enough.
Now hopefully someone with a better education and its accompanying vocabulary
will provide us all with a proper, scientific explanation.