He is growing into a big boy. You should be excited he wants to protect you, its out of love he does it.
While the noise he makes may make your skin jump thats the same feeling hes having too. His dog sences are kicking in saying danger danger. As for the lady in the car next to you, Im gonna guess she either had bad taste in music or cars....
In all seriousness. They believe in a boundary area or zone so to speak. Cross that line or get in that space its game on.
I have an extremely protective pack here at my house. One sounds the alarm and they all go into ninja mode. But I have to agree that when there in the mist of sleep and something sounds the alarm they have to check the source of the noise before we can calm down or go back to sleep.