I wonder if any kind of shrubs can help absorb or block sound?
I know it sounds kind of silly, but talk to an actual nursery and see what they say. If there is something that will work, I am sure you guys could do the work yourselves to save money, though trees/shrubs are not cheap either.

I would also suggest getting in good with the local PD, invite them over to watch and give them a soda or bottle of water. You want these guys on your side, as if it gets ugly, I am sure some of the locals may bring shovels down there after dark.