Ironically, I just watched a PBS special about wild horses in which the foals did that very teeth clicking thing to the older horses as a sign of submission. It was so neat because I had just read your post.
Yup, I've seen that with most the little ones around here. My stallion sometimes runs in an arena next to a bunch of yearling colts, and every time they touch noses all the babies do the teeth clicking thing. My gelding Charm did it when he was first meeting the mares he lives with now. Since he was one boy raised by 3 moody girls, he's pretty well gotten the picture that he's always at the bottom of the pecking order.
......so now even though he's a bigger stronger 2 year old, he still does it to every horse he meets!! Horses are funny.