I think it's the deep noise of trucks/tractors that scares him, but he's not at all bothered by other loud noises. It's strange. He's VERY reactive to scents; if something comes into the house that "smells funny" he goes off.
I've had him since he was 8 weeks. He's now almost 3 years. Some things have improved in that time - he used to be terrible with resource guarding and now controls it much better - but other things got worse. He wasn't always reactive to strangers but he never did like them much; he latched onto me when I first brought him home and refused to interact with anyone else for a good three months (which did wonders for my anxiety
).
He's had private training, group training, and multiple consults. I've considered taking him to a vet behaviorist but would have to drive 3 hours and am unsure if it's worth it or not. He takes flouxetine and trazodone but they help only minimally. It's hard because he's super duper reactive (barking/growling/lunging), has almost no impulse control (we're working on that one), but it's also very clear that these things make him upset as he will seek close contact/comfort and will cry when these things bother him.
He was a rescue dog and was pulled from a hoarder's house and put into the pound, then rescue when he was very young (like 6 weeks). There were SEVERAL other dogs pulled as well from this house. I'm guessing there was a good deal of inbreeding going on, but additionally I think he's just got a genetically weak temperament. On top of that I think life was very stressful for not only him, but his parents as well, and I've read that severe stress within the womb can cause all sorts of things to happen developmentally too.
Probably the hardest thing with it, though, is that he's so smart and so incredibly sweet. He's a Cocker mix and I've known some Cockers that were just nasty dogs in general - my Chisum isn't, he's just so scared of the world and figures a good offense is the best defense. He's so very loving and he tries so hard. If I ever had the chance to meet the woman who hoarded his family and brought him into this world, I think I'd punch her right in the face.