Orthotrope

Hi all, I just got a brand new UM2 extended. During the third print, I noticed ominous noises when the X-axis (the one from front to back is the X-axis, right?) was moving. It's a crackling noise that is also there when the axis is moved by hand and corresponds to about every half-turn of the drive shaft. See the video below - very pronounced towards the end. I checked as much as I could without taking anything apart yet, do you guys have an idea what could be causing it?