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.
Looks like it is only live view specific issue. I did few more tests keeping eye on distance scale. Lens moves in small steps/increments in live view instead of one go focus movement. Each step produces this tuk noise. It also makes few more steps back and forth before the final focus confirmation. All this tuk tuk noise comes from this small step moves. Some times, this operation takes for few seconds and makes more tuk tuk noise. Probably, it moves in one go during phase AF instead of these steps
Other than this annoyance, live view with tilt screen looks like very useful feature for static subjects. We can focus any where on the screen instead of fixed 9 focus points with phase AF.