With your phone unlocked, screen on, put your ear to the Touch ID. Makes a curious clicking/ticking sound. Confirmed on both my iPhone 6 Plus and the wife's iPhone 6. Does not do this on an iPhone 5s. I'm wondering if this is the circuitry being kept alive for the Reachability feature? Either way, the only reason I discovered this is because I could hear it in a quiet room when I rested my finger on the Touch ID ring to log in to 1Password. Yep, hearing like a dog!