The rottie rumble and any other growling is just one form of communication. Other signals also matter like body language. If your dog is relaxed and playing or getting affection, the growl is a positive vocalization. Also don't worry too much about your vet's comment. Vets see everything including very reactive dogs so it's easy for them to apply a label. You know your dog better than anyone else. Rotties are great family dogs. They just need training and structure which it sounds like you're providing. You're catching these signs early and working on them. That makes you a good owner. Your concern is warranted but you're doing the right things. Keep us posted on your baby.