Not 100% since I'm not in the car and these sounds are pretty faint but I think there may be a noise associated with each function, more multiple clicks for steering and another click-like noise when the brake applies.
The sounds seem like they are coming from near the footwell area. It is a muted click sound about a quarter as loud as the turn signal indicator or like a muffled popping bubble wrap sound. I routinely get about a dozen of these clicks in the first 15 seconds of driving or so -- e.g. when reversing out of a parking space, putting the car into to drive, and then driving down the driveway and turning onto the street. Then regularly while Pilot Assist is on. I'm sure with any other car, I would never notice it, but since the XC-90 is so quiet, they are hard to miss.