I'm so glad to have read this post. We recently got an 8-9 week puppy and he's a little firecracker. My going on 5 yr old GSD is as patient and calm as the day is long but the puppy is constantly running at her and nibbling any part of her he can get his hands on. At first he was only going for the stomach (which appeared as if he was trying to nurse) but since then loves going for the legs,face, ears etc. I know it's all puppy play, and I occasionally step in when I can see my GSD has grown tired of it but for the most part I have let them do their thing. My older GSD treats the puppy exactly as you describe (showing teeth, large open mouth, low minor growls) but has to this point only given a soft nip on the puppy's ear when it has become too much. I assumed, as I feel like I know my older dogs demeanor, that they would work it out together but my girlfriend has constantly been nervous the older dog would take it too far. It's good to see others have witnessed, experienced the same thing. I believe I will use it as a training tool as well! Thanks for the advice.