I wouold speek w/ your neighbors. They should understand especially if you are friends & they should appreciate that better than the police showing up.
When I first got Raustyk she would bark constantly (thank god it was only during the day while I was not at home) but I had no clue until the cops showed up. The cops told me my neighbors complained over a couple months yet they never mentioned it to me. I would of loved to be told first, how was I to know as it only happened while I was not there. Thankfully the cops were understanding as I told them I never once recieved a notice of any kind, and I was a trainer & vet tech at the time. I finnally got her to stop but it took a long time & many many different tries w/ different products/situations/training.
Sorry of the blabbing but anyways I would speak w/ them, maybe they are heavy sleepers & have no clue their dog is barking either! Good luck!