I have had 2 dogs that do that. My GSD (rip) and my rescued boston/pit Libby.
Both of them high energy/high strung dogs... and they did it in situations where they wanted something or wanted to do something that they had to restrain themselves from. (Libby? Self control?
) LOL
Libby also does it to sour or tart foods she likes (like grapejuice or lick of a Jolly Rancher) But she has a small cleft that I think effects her taste and smell (possibly for the better... I am observing this possibility, but its hard to tell).
Here is a picture of her smiling. You can see her harelip. Imagine those chops chattering. It doesnt make a noise, but its really cute!
[img][img]http://i42.photobucket.com/albums/e347/pitpack/cheese.jpg[/img]