I have a boxer mix that will sometimes growl and look ferocious in her sleep. Her eyes will be open, but she doesn't really focus on anything. The first time she did it she was laying on the sofa next to me and I thought she was awake and growling at the cat. I finally realized that she wasn't fully awake and was still dreaming. She has done it a couple of times. I just call her name softly and rub her on the rump until she wakes up. There is a physical change in her posture between her being half asleep and fully awake. There is a definite moment when she wakes up. Then she is back to her normal sweet, goofy self.
Well, collies make an incredible amount of noise when they sleep. Mine sounds like he's dying half the time he's asleep, it sounds like the demons of He** are being unleashed.
There are a great many possibilities. Someone passing by in a car, mice in the walls, the dog had a seizure and felt weird when she woke up, there's a million things. The dog could be in a lot of pain from swallowing something or being ill.
Our BC Zooky growls in his sleep. We figured he was dreaming. Also has warned us about Brown Bears in front of our camp before any of the rest of the camp could hear them.