If it is knocking loudly, I would not drive it or start it at all. The more you run it the more damage is being done to the crank and the rod. Sometimes you could go for months with it knocking, sometimes you could drive it a few miles and put a rod through the block. It really doesn't matter what exactly it is at this point, if you unplugged the #5 plug wire and the knock went away, the engine is going to have to come apart. If you keep driving it you could potentially make the rebuild a lot more expensive. Consider also that all the missing bearing material from the bad bearing may be getting circulated to the other bearings, cams, and the turbo,