Well, a rod bearing would be pretty destinct sound but as a rookie you may not know what to listen for. When i first started building motors I thought the same thing when i heard that sound. I always went to the worse case scenario cuz thats just my luck, but that wasn't the case. You are correct about the Plug scenario. I don't know about the crack causing the noise but a dead cylinder can definitely cause some knocking sounds.
As an begginer, how much engine work expierience have you done? I want to know your expierience with timing? Was the distributor tightened down when the "knocking" began? What kind of ignition i on the vehicle? If its HEI and the dist. has gotten bumped to far advanced or retarded it could cause Detonation at the wrong time and create this sound. That is why the newer engines have computer controlled ignition/timing systems equipped with an anti-knock sensor the allows the computer to advance or retard the timing to prevent detonation. I would definitely look into this before you go pulling anything apart. I have made the same mistake, I thought the worst, but it turned out i just had 5 dead cyinders, or the timing was way off. Hope this helps,