I have the same rear axle noise on my 2017 Bolt, and it is very low mileage at ~9,000. The service department took several days to isolate the noise via largely a process of elimination, swapping out many components in the process - when they finally identified it, they brought me into the shop and demonstrated the clunk while the car was on the lift. The axle is designed to flex. By applying upward pressure (a jack) on one side of the axle, they can replicate clunk/click - it comes directly from the axle itself. They speculated that I probably heard the sound most often when one wheel goes high, torquing the axle vertically - running over a bump on one side, for example. Insofar as I can tell, that is correct. While the service dept does not believe that it presents a safety issue (after all, I have the "clunker" back, they have ordered the replacement to be installed in a few days. My car was the first one into the dealership with the problem, however, during the remainder of the week, 3 others came in for the issue.
They pointed out that since the Bolt is so quiet, and since the sound insulation is less than most cars, noises like this are more perceptible. But there was no hesitation on their part to order the replacement since they don't have enough history with the axle to know if it would pose a downstream hazard.