Inertia for low RPM quietness and inertia during high RPM quietness. Trust me its Knocking at those other RPM's it just not at a frequency that generates an audible noise.
Knock on anything (like your desk) with your knuckle then with your palm what you are hearing is a change in frequency. By changing the shape of your hand you change the frequency generated.
Since the mass of the piston and the rod never changes, that magic RPM range that generates the "knock", happens to be its natural harmonic (the frequency it comes alive at).
Ever see an exhaust heat shield missing a bolt vibrate like hell at a certian RPM? That is its "natural frequency". By adding that missing bolt or even by hitting it with a hammer and bending it you change its natural frequency. By bending it you made it stiffer (most likely) and it will vibrate (come alive) at a different RPM. If your lucky enough your engine wont go to that RPM and your hammer fix worked. If your smart and you put the bolt in hopefully early enough that the heat shield didnt crack around other fasteners.
Hope that answers it.
I have done this job without ever taking the head off, its easy. Just (remove the oil pan first) rotate the carnk to the lowest position for that rod. Remove the cap, remove the bearing in the cap, place the clean bearing (old bearing) on the crank and rotote it around using a screw driver in the little oil hole. This is just to loosen the bearing. Remove the bottom bearing the slide the top bearing out. Lube a new one (with synthetic oil is my preference) and slide a new bearing in. Make sure that they go in the right direction and are centered properly. Install new bearing in the bottom (rod cap) torque to spec and continue on to others. The whole job will take less than 3-4 hours. Might as well do the Oil pump shim while your at it.
