Bitter learning curve. Absolutely DONE with carburetors!!!

I had a simular noise as you are describing on a fresh 340. What I found was the crank thrust bearing wore and the crank counter weight would move against the block. It was getting worse that is why it came to the shop.

We tore it out and found the rest of the bearings were fine. The knocking was a slug of heavy metal that was used to balance it. The weld was protruding from the side of the counter weight. As the oil got thin the crank would move causing the slug to hit the block every time around.

Grab the front of your crank and see how much play it has front to back using a dial intcator. Take the plugs out and turn the crank pushing front and then pushing rear. You should be able to feel it hit in one of these directions turning the motor by hand

We grund the weld and installed a new thrust bearing. Noise was gone