I also have a year old German Shepherd who had a big issue with wanting to defend the house from people coming to the door. He's not a biter or growler, but his hackles would go up and he would bark just like your guy, if I allowed him to he would've probably busted open a window to get to someone outside or to another dog.

Have a friend come by and enjoy some beers in the garage or somewhere where your dog can't see him while inside. About 3-4 times every hour, go inside, play with your dog, get him comfortable, watch TV, and then have your friend knock on the door, make a ruckus, get your dog acting like this. You can work on the quiet command while you open the door for your friend. Make your dog sit and remain still and have your buddy give him some of his favorite, high value treats. I used some hotdogs, whatever works.

Keep reinforcing positivity from people approaching your door, let him know unless you direct him that there's a threat, that there's no need to get worked up. Once he's calm with you, you can switch it up to leaving your dog completely alone and having your friend come to the door and reward him.