In response to your reply;
 No BG, but that "unusual noise" was present. Rather than calling our local PD, spending 2 minutes on the phone describing the situation (before they dispatch), the 45 second delay (at least) before dispatching and their average 9 minute response (dispatch to arrival) I decided to look first. Had it been a BG, (a)it's doubtful they would have heard me coming, (b) I did approach the area cautiously and (c) my 3" .45 wheelgun would have evened the odds considerably. I thought I had a BG when seeing the hood open, but my neighbor's build is easily ID'd even at night.
 While he didn't go outside on this incident, he has had to do so on others. Missouri has a terrible meth problem and lots of property theft from yards. If it ain't nailed down, they'll find a way to take it. When his garage alarm beeped one night, circled wide around the garage with his Glock. He found a young couple trying to load an old Pontiac engine block someone else had dumped on his land. He would have interrupted them, but they were saving him the effort of recycling it. Once they got it into the truck, he let 'em leave and reported it. Kids picked up the next day at the recycler nearby.
For the most part, I agree that one should call the PD once it has been established that there is a strange noise outside. This works mostly for suburban and urban areas. But where you can't see common areas (as in my case) I think it's prudent to at least try to look/listen before calling the PD. In rural areas with long response times, reliance on the PD to provide any real assistance or deterrence isn't prudent.