Why is my cat afraid of certain noises?

He's afraid of the sound of plastic bags, trash bags, and spray bottles/cans. It has been like this since we first rescued him last November. Whenever he hears any of these sounds, he immediately darts off. For a long story short about him, we rescued him from an abandonment situation, he still had his tags any everything. All attempts to contact his former owners have failed. So we took him in The vet estimated him to be between 6 and 8 years old. He's a very sweet cat, just afraid of certain sounds. He's also afraid of the sound of one of us opening the silverware drawer. We don't know for sure what happened in his past, so that might have something to do with it. Any help is appreciated! Thank you!