You would mostly hear when accelerating after decelerating like when going around corners. Once it engages, it stays that way until you let off and it disengages and "free-wheels."
Most swear by the locker, but I just have a regular posi in mine and it's quiet. I seem to remember Royce talking about a "soft locker" or something along those lines from Summit. I also recall that it was rather pricey. He'd be a better source on that unit.
Mark