The cartridge body is secured by a bolt at the bottom of the fork legs. The fork cap holds the top part of the cartridge (rebound rod assy).
This video demonstrates where the the "bolt on the bottom" is and how to get to it. Skip ahead to :45.
https://www.youtube.com/watch?v=5A0p2OH6FmI
I'd doubt its the fork cartridge (but it is possible). It could be worn bushings, especially if the fork oil doesnt get changed regularly.