There are two serious knocks that I look out for. A main bearing knock has a bit of a thud to it, it is sort of heavy sounding. A main bearing will usually knock under load at low RPM. A rod knock is a harsh sound , rather metallic. It is most noticeable when you give it a quick rev. As the engine is between accelerating and decelerating a rod knock appears as a rattle. Either knock is a bad thing and shouldn't be ignored.
The rods have shims that can be adjusted down. The procedure is to remove shims on a rod a retighten until the rod hangs up and can't be moved back and forth. Then one shim is added back.
After taking off the pan, It would be a good idea to look at each rod bearing babbitt surface to see how they are. Sometimes the babbitt drops away from the iron surface and causes a bearing failure.
The main bearings had shimms too, in the original configuration.
