When the wishbone first starts going off, you're still pretty far, so the BPM is really slow. So slow that I never just stayed in one spot listening to it. When the pitch went up, I thought I was getting warmer. When pitch went down, colder. So I kept moving, and the pitch went up and down such that I'd try to find the highest pitch I could, and start digging.

The pitch goes up and down even if you don't move. It has nothing to do with anything. Doh. BPM. Got it. Found the silver no problem...