Waymo spokesperson Chris Bonelli explains why the problem started and why its autonomous cars are quieter now (shown in the livestream below):

We recently introduced a useful feature to help avoid low speed collisions by honking if other cars get too close while reversing toward us. It has been working great in the city, but we didn’t quite anticipate it would happen so often in our own parking lots. We’ve updated the software, so our electric vehicles should keep the noise down for our neighbors moving forward.