Hi @Brenna , one quick idea, if you’ve not already mentioned this, is to see if a white noise machine, or radio or background TV will help to mask some of the outdoor sounds.
If the dog also has visual access (i.e. through a window) for watching delivery trucks, that can further exacerbate things. If that’s the case, I’d block that with opaque window film or poster board.
Does the dog’s condition of “high alert” translate to a very anxious dog? If so, a discussion with a vet who specializes in behavior meds could be helpful.
No foods, even fresh proteins like steak or roast chicken, will get the dog’s attention? If not, perhaps having a leash dragging on the dog (under supervision) will at least allow the pet parent to move the dog away — on the first bark — to a quieter area of the house so he’s not hearing the trucks and, importantly, not hearing them drive away. I worry about a dog possibly thinking that his barking drives the ‘bad trucks’ away, thus reinforcing the barking.