If it was still in running condition, you could check a few things to narrow it down, such as pulling individual spark plug wires to maybe isolate the knock to one particular cylinder. I assume they checked the oil pressure when it was fully warm and it was not below specs? Bearing knocks don't normally get better when the engine oil warms and thins out. There are all sorts of knocks that have different sounds and rhythms. Since it's already out and ready to be torn down, just do it and inspect every part. You should be able to find the answer at that point.
