Yes, for one, using the search bar to look for knocking will bring you to the problem and the solution. The knocking has to do with the bearing on the end of one of the rails moving inside the housing that attaches said bearing to the printer wall. Essentially the bearing is moving back and forward knocking the frame causing that sound. It should not cause any problems with the print, I have the sound on my personal machine and it can run for days without issue. It is a slight annoyance but nothing more. At lease that is what it sounds like to me over the quite loud fan noise.
(upon using the search function I was unable to find the problem so simply as I had stated, I had found it before by the same means so something is up) I apologize for the remark.
I contacted Zortrax support team directly and they sent me a .PDF of a service guide, but wasn't the solution. I also reluctantly wiped down all the rods and re-lubed. Reluctant because the printer only been in service for 20 hours approx.
But sounds like you have the answer. I will take on board what you have said NS, Cheers