John has gotten the first item that happens at nearly every startup or shutdown. But the brake system might also be doing a pressure adjustment or air purge after you power down. A pressure adjustment sounds like a repeated cracking sound. Kind of a very slow buzz. It's pretty loud. If you hear a hissing noise, that is the brake system purging air from the system.
I have heard the break system crack/buzz while coming to a stop and when the car is shut off. I have heard the hissing noise only when I was out of the car and it was powered off. I cannot confirm that the air purge doesn't happen when running. It is too soft of a sound to be heard while running down the road. The crack/buzz does occur during regular driving.