In the end, it turned out to be nothing of the sort. It was a yakiimo truck selling roasted sweet potatoes! I ran down and took the video above (which is very poor quality, but can give you an idea of the sound we'd been hearing), and then bought a sweet potato for Chad and I--a whopping 500 yen!
The price was worth the experience I guess--it was cool to be able to take a picture of the guy's truck and get a sweet potato randomly at 10:00 at night. The guy even whistled to his dog to come to the front window! Then again, Chad pointed out that by buying a sweet potato from loud truck-on-the-move, I had also encouraged the guy to repeat his sales on other nights.
This is the back of the truck where the roasting is done--and where all the noise comes from!
Our great, big sweet potato
Delicious as the sweet potato turned out to be, I sure hope Chad isn't right about that noisy business coming up our little mountain again!