Broken crankshaft?

The mileage is low and I would go for a defect in the crank. I sounds more like a piston problem than a crank. The piston arm could be loose on the crank causing the racket. I would wonder about getting a short block which is the engine without the head and thus new rings and bearings. See how that compares to a used engine. The rebuilding of yours is a possibility however, it is also possible that damage has been caused to the block as well. Did you ever have the engine oil drop really low?

I do know that the sound would be loud with the slap of the piston cap being loose or worn. I have seen a vehicle with a crack crankshaft that ran for another couple of thousand miles but the break was at such an angle that allowed it to contine to rotate. However, the noise that engine gave was not loud. And knowing that the metal on the piston is probably aluminum versus the crankshaft which is steel it is likely that no damage could have been done to the crank. I would sure want an honest mechanic to do the work and only put in what is needed. Same principle does not apply to the block as that is probably aluminum alloy.