Joe, I wonder if this is like the 99 that I have on my bench right now. It's been trying to tell me knock knock jokes, but never gets to the punchline. Worse still, the noise seems to rocket around the machine - sometimes it seems like it's by the needlebar, sometimes by the vertical shaft, sometimes even by the hook. As with your machine too,.. if the stop motion knob is in bobbin winding position, no more knocking.
I finally narrowed it down to the whole horizontal shaft. It was moving left to right, there's a tiny bit of play in it, and when you get it up and running, all heck breaks loose. The faster you go, the worse it gets.
Might not be the same problem, but maybe a direction to troubleshoot in?
The 413 might have a couple of nylon washers to take up "lash" (I think that's the word) and quiet them down. That 758 I was working on? I was oiling near the cams, when out of the corner of my eye I saw something roll casually toward the back of the machine.... it was one of those washers, broken in half.
Since you have two of them there, maybe you can compare the two to see if there are washers in one and not the other, or something else that's obvious, or not so obviously different?