I am in the same camp as Old Ugly. The rebuilder should take a more active interest.
Here is an idea: Make a video of the knock and send it to the rebuilder.
Since you pulled the distributor and replaced it and the knock happened afterward, then logic would say it is related to the distributor. If the screw that holds the distributor in the engine is too tight, it can bind the shaft and cause a knock. Look there first. You can pull the distributor back out and try turning the shaft with your fingers to see if it is binding. Look for witness marks on the shaft that might indicate where the problem is. If the distributor was hard to push in, there is something wrong with the shaft. If you have access to another distributor, you could try that, but you would have to time that distributor.
Of course there are a bunch of other possible sources of a knock, but the evidence points to the distributor.