I have had similar issue with Hercules. One day after playing with his frisbee for about 10 seconds he suddenly lost interest and limped off. I approached him a went to look for the reason he was limping. As soon as I touched his right hind leg he let out what sounded like what you described. I have never heard a dog make that noise before. I was obviously concerned so I watched him all night and took him to the vet that morning. When we got there he had no symptoms, but the vet mentioned it is possible for them to get strained muscles and cramps.