I have had a clicking with my TAZ 4 since I got it, and I have updated it to a Hexagon hotend which comes with a new extruder and all.
I finally decided to figure out what the clicking was as I have read of other people mentioning the same issue. Turns out the large herringbone gear is ever so slightly out of round (mind you these both came from Lulzbot). When the machine would retract there was a click sound if the motor was on the smaller side of the gear. I could see the exact problem with the extruder in my hand. On one side of the herringbone the smaller gear was perfect, not tight, not loose, but on the other side of the large gear there was about a 1mm gap in the teeth so when the direction was reversed it would smack into the other side.
When I decided to figure it out I was printing out a vase that had a tone of retraction and that is when I figured out it only did it when the gear was on one side. Basically it seems to be about 1/4th of the gear is where the main issue is.
I am now printing a new gear with a little bit higher resolution than the original. I plan on taking a lot of measurements to assure it’s accurate and then try it to see if the clicking is gone.