Quicky media update
I'm curious about the ending... did the foghorn serve as a literary device, an acoustic strikeout of information that the characters cared about but the story didn't (since they were only talking about the McGuffin)? Or did it serve to make the ultimate outcome uncertain? I'm actually certain that it's the former case. It's an intriguing stylistic technique I don't think I've seen much (the playful name-bleeping in Kill Bill is not the same thing at all, being a very deliberate tease), but I bet it confused a lot of audience who thought it served the latter purpose. (And I could be wrong. But I don't think that I am.)
SPOILER... HONK
So the first thing I thought was: shit, things were going his way for a second, and then dumb luck ruined everything and now the process is lost, since the Feds will hear only the horn. But the characters' subsequent actions (once the bad guys are put down) have an air of satisfaction about them, implying that all was secure. So, it took extra film content for me to understand that trick as it was intended, which is why I didn't like it too much.