With mine she rolled a noisy cart past us and opened an umbrella kinda abruptly close to Gus. But I think it can vary. It's acceptable for the dog to notice and perk up at the noise but not freak out on leash or anything. I think its okay for them to flinch though
In ours they dropped a metal bowl and a couple plastic ones near the dog. There was also someone pushing a wheelchair past us.
I don't remember how they're "expected" to react. Neither of mine flinched or jumped. I think they can acknowledge it but they're not supposed to be scared or freak out. Fiona just looked at the bowls when they dropped and continued on. Tali smelled the bowls and then tried to pick one up and give it to me (she was in rare form that day). LOL
Both my girls just glanced at the wheelchair. They've both seen wheelchairs before; no big deal.
as far as "what" they can be, it is pretty much up to the evaluator. It should be something designed to startle most dogs. Loud noise, sudden movements, things like that. The dog is allowed to become startled, but the dog has to recover and go with the owner. Dogs aren't allowed to try to run away or become aggressive.
Our boy Rush just passed his CGC about a week ago. His distractions were clanging of 2 metal bowls, someone running a shopping cart past him twice and the last distraction was walking among people with and without dogs. The trainer advised that they can be interested in whats going on but not have a physical reaction to the distractions (she gave examples as dog trying to pull away or hide behind the handler).