my dd used to do this. I thought it was her way of settling without having to feed or suck on a dummy a sort of simulation sucking. She also used to do it as she was drifting off to sleep up until she was about 3.
My DS has always does it, and at 6 yrs old still does is when falling asleep and while he is sleeping. It is a sort of unconscious sucking, I jus put it down to a sort of comfort thing like sucking a thumb or dummy.
Oh that's such a relief -- thanks Kittybrown and Overmydeadbody! Come to think of it, my little one is a rather 'sucky' baby likes to be comforted, so it makes total sense! I was just worried because I read quite a few threads about clicking and thrush, and was just worried it may be an indication of some sort of illness?!?!!!