It sounds like he is barking because he is afraid. To help stop the bark we have to teach him that he doesn’t need to worry about those sounds and that they are no indicator that something bad is going to happen. To do this, start with the sounds you can control, such as a knock on a door. Softly knock on a table or wall. IMMEDIATELY start praising him and give him a treat. By reacting with praise as soon as you knock you should be able to reward him before he barks. If you miss the moment and he starts barking, calmly and quietly say “nope” and turn your back to him. When he stops barking, face him again and try to knock softer or do just a single knock instead of several. When he is quiet as you knock—PUPPY PARTY! He will quickly learn that if he is quiet when he hears the knock then he will get treats and love from you. As he gains confidence then you can start knocking louder and louder until a knock at the door makes him run to you for praise instead of bark. You can apply this methodology to other sounds as well. Does he bark when he hears people walk by? Enlist a friend to walk back and forth in front of the house as you praise and treat him. Does he bark when he hears the neighbor come home? Have your neighbor text you when he is on his way home. Be ready with treats and as soon as your dog alerts to the neighbor, PUPPY PARTY! Additionally, make sure he is properly exercised. A tired dog has less energy and desire to be on guard at home. Finally, make sure to get him out and socialize him if possible. These life experiences will show dogs that the world is a safe place, and will help them feel safe at home.