Possibly Found: YA book from the 70's

Posting this one for a friend:
Book would have been from 1970-1974 range. It was about 3 teens, 2 who were possibly brothers who lived in the southwestern US in mountains. There were lights or noises from caves in or near the mountains and the boys investigate and find that someone was making the lights to frighten anyone who came near because they were stealing silver (Or some precious metal) from the mountain.