My guess is that the yaw damper part of the AFCS isn't tuned exactly right. You can see this in various ways on lots of aircraft - look at the wingtip, and if it's moving back and forth every so slightly, or rolling, then the yaw damper isn't working correctly.
The yaw damper is put in to correct / stop Dutch roll, and if it's slightly out, the result will be a small (hopefully) oscillation in either yaw or roll.