First off, great job for being consistant and NOT comforting him when he reacts to the loud sounds. As I'm sure you know, that would only enable the behavior.
Could I ask what type of collar you're using on him? Because he pulls and reacts to noises, you want to ensure that you're wearing a safe collar that won't hurt him, and also won't allow him to get out of it.
What exactly is his reaction he has when the loud trucks pass? (Whining, putting on the brakes, leaping, pulling back, tail tucked?) You mentioned it's some small trucks, but does this behavior occur basically when any or the majority of vehicle passes? Does he exhibit this fear when the vehicle comes from behind you, or even when it's coming towards you?
Have you been trying to take him on the same paths every time?
You mentioned that he used to pull a lot when you first got him - how long have you had him, and in which direction was he pulling? Did he pull to lead, or pull back to escape? When he first pulled on the leash with you, did you notice this behavior towards the vehicles at that time as well?
Sorry for all the questions, just want to get a better idea of what's going on so we can help you.