I would say I found a band-aid vs. a fix. 
No root cause…still going back and forth with Creality support. Currently waiting for their reply after I let them know the printer frame is perfectly square and that I have not “hit” the printer or done anything to the belts. They actually asked me this…although I don’t think they meant that literally.
In any case, the printer is only about a month old and I’ve done nothing to it but print.
Darn im sorry man. I hope you manage to get that fixed
So here’s an interesting update…
Creality ended up sending me a new motherboard. I watched the tutorial to swap it and well, it’s not terrible but still a lot of effort for something I honestly don’t think is going to fix the issue. In fact, it seems more likely I’ll mess something else up in the replacement process. I ended up just deciding to leave it as-is. In the meantime, for some unknown reason, just like the knocking started, it has stopped. I don’t know if the printer somehow recalibrated itself or what but when it homes, no knock in either direction.
Bizarre…but I’m happy.
Hello SJMarty,
It is good to see you are happy printing now !
There is one thing that crosses my mind often when unusual problems arrive for some printers and can’t be explained or found and that is “mains power”.
Now, not everyone can afford a beefy UPS to run their power hungry 3D printers from, but I can bet that it would most likely fix a few that have odd problems.
Things could cause mains power (240 or 115v AC) to either have spikes/brownouts/etc etc like for instance, beefy motors/pumps, welding/all manner of things possibly happening next door/downstairs/up the street etc.
Are the power supplies that are made for the printers we use built for these possibilities, or made to a price ?
Actually, if you have access to a DSO, check what the mains looks like.
It SHOULD be a sine wave … it most certainly ISN’T a clean one !!! all that noise generated by everything we use around the house is amazing.
Cheers.
That is, as long as you KNOW what you are doing and not going to kill yourself trying.
In fact, you don’t have to get close to mains at all to see it on a DSO, just hold a probe and adjust the voltage per segment to 10mV at ~.2ms … … …Ha, so much for sine wave