Yah. Focus ring is not smooth. It makes some sound during AF. Sound it makes during live view is a different story. Mine makes very loud sound with Live view focusing (Grinding sort of). I am going to check with Sigma and see if it is expected behavior.
According to Sigma's website, it defines HSM as "HYPER-SONIC MOTOR
This lens uses a motor driven by ultrasonic waves to provide a quiet, high-speed AF." Mine has the same sound intensity levels in both, on and of live views. I have seen here there are many who have the same problem like this. I wonder how Sigma would respond to the question on AF sound if everyone called to ask.