What is your reaction when he hears these noises and gets scared? Don't even blink an eye at him or give him attention, act like you're ignoring the sound. He feeds off our emotion so if you're scared, he's scared, if you don't seem to think it's a big deal, he shouldn't either.

But since the fear is already in him, make the loud noises associated with something GOOD. Like every time you're out walking and you hear one, stop and give him a treat (as long as he isn't freaking out, because he may think you're awarding the behavior).

EDIT: Forgot to add... I saw this EXACT scenario on this dog training show once on Animal Planet... except the dogs were hunting dogs. They got a recording of a gunshot and would play it starting the volume down low and slowly raising it over time. He was rewarded for not freaking out. Over time they would just play it while they're home, but if they left they would play it softly when they were out of the house so he would be used to it. Eventually he was just fine when his owner shot a gun standing next to him