Just a moment ago I laid under the table and watched the movement from there, after loosening some screws and the belt a little bit.
I noticed a gentle “bang” which ist reproducable.
It’s the same movement from a slow to a faster part of the pattern.
I can’t find the source of the noise but this could be the problem. I assume it comes from the X Power Corner when only Y is moving and then X starts again moving.
I found this thread which sounds like a similar problem.