Knocking sound from flex plate??

[Brewtal66]

Backstory: my buddies truck needed the engine rebuilt. We pulled it, stripped it to a long block and the engine shop did their thing. The engine runs great, but has developed a knock. We took the truck back to the machine shop and they looked it over and said that it's either the flex plate(bent) or the torque converter.

Does that seem right? Would a torque converter or flex plate knock like a rod bearing? I'm racking my brain trying to figure out how a possibly bent flex plate or bad torque converter could cause a knocking sound. Mind you, the transmission works flawlessly too, so I'm thinking if it is was he's suggesting, it would have to be a bent flex plate.

Thoughts/ideas?