CMD, I have the same situation. The young dog doesn't seem to care that much, so far. Bella didn't care until we were on the street and a huge branch fell out of my neighbors tree, which broke branches under it. I barely heard some noise, but Bella heard it and was looking up in that tree. Next thing you know you can hear a very loud cracking noise and Bella started running. I think she ran before the boom when it hit the ground. She wasn't thinking about me, and when she took off and I was standing there, it made me almost hit the ground. I was being pulled so hard I lost my balance and was forced to do a few running steps. So I had to let the leash go, or I would've been eating pavement for sure. Luckily this was right in front of my house, and Bella ran straight for the front door and waited on me. Since then loud noises scare her. She has heard trees falling in the woods before, and she knows a boom follows if the tree or branches are big enough.
When she was a few months old, I took her to where some guys were target shooting, just so she could hear loud noises and not be scared. I took her off a piece, and said go ahead. So they started shooting at targets. I walked Bella a bit closer and closer. She looked that way, but wasn't scared at all. Now? I think I know that answer for sure.