Growling is natural.
It's either for play, fear, lack of confidence, stress, or to give a warning.
If your puppy is growling during play this is completely NORMAL and A NECESSARY ASPECT of their NORMAL GROWTH.
Never discipline them for growling as you are telling them it is not ok to be stressed, offer a warning, or communicate during play. This is very unfair and confusing to your dog.
Dog's that learn to not give a warning, will go straight to bite instead of communicating their discomfort first.
Figure out what is causing the growling and if it is fear, warning or stress then learn from this experience and simply remove them from the stressful situation.
If your dog is growling during PLAY, it is PERFECTLY NORMAL.
Bottom line is respect your dog's communication system and allow him to use and develop his native language.