I've owned my Sonic for a week and experience a single clunk whenever the car begins moving after being off. I assumed this was the ABS system engaging since my '93 Miata used to have a similar noticible engagement upon rolling from a start.
I've had cars with ABS before and I don't remember them making that noise. It hasn't made the noise for a few days, I wonder if the cooler weather has anything to do with that.