Timber has never growled at me in a mean way, but when I am training her and she is getting fed up, but will let out this growl/moan type thing as if to say "I'm tired of this, I need a break."

I had a rottweiler, Zoe, who used to growl at me in a very aggressive way. She scared me. I'm sure that if I were to growl back she would have attacked me. She was very dominant and saw me as a threat. Luckily whenever she would start to growl at me, Leather would attack her. Leather was boss, and Zoe knew it. Leather would have protected me from anything.