After some years, my IKEA Markus chair developed some clicking/knocking noise whenever I swivel in it. This was very annoying for everbody in the same room.
All posts and videos I found about problems with these chairs were talking about re-greasing the swiveling mechanism under the seat. However, there were a lot of people that also reported, that that didn't work for them.
After digging some more, I finally found this video. This demonstrates how to disassemble parts of the gas cylinder. In there, you'll find a few washers, a ball bearing and a rubber stopper. In my case, the washers had deep grooves in them and the shavings were clogging the ball bearing.
I cleaned everything up, greased the ball bearing and assembled it with the washers upside down, so the sides without grooves are now pushing against the ball bearing. And that did it. I now have a quiet chair again.
nb: At least in Germany, IKEA Markus chairs have a 10 year warranty. So you might as well just return it and get a new one.