L.P.
I'm thinking this isn't really that big of a deal. Really, there are so many more unpleasant things that he could do (and of which he would probably start doing as soon as you "correct" the growling behavior). Chose your battles, mamma. After all, everyone has to express thier anger somehow. Just validate his feelings and give him words that he could use to express himself. Like saying " I can see that you feel angry/frustrated." Someday, his verbal abilities will catch up with his emotions and he'll use words to express himself.