Dogs communicate in many ways that are too subtle for the average human to notice. Yawning, lip licking, and looking away are a few signals of anxiety or discomfort. Unfortunately, humans don’t always respond appropriately to subtle signals, so dogs sometimes find themselves having to escalate to a growl to let us know how they feel.
Here's the bottom line: Punishing growling suppresses the growling, but not the anxiety. Punishing growling eliminates a useful warning signal. Punishing growling may lead to a dog biting "without warning." Never punish growling. If your dog is growling, you need to put a training plan into action that addresses the real problem (the anxiety your dog is feeling) not just the symptom (the growl). If you don't have the expertise to come up with a training plan yourself, find a certified professional to help you do it, so everyone stays safe.