Our Rott does the same thing
We have a 1.5 year old rottwheiler and she growls when being petted. She growls when we hug her, pet her chest, and pet her back. She will growl as she climbs up in our laps. She is spoiled, but we have had two other rotts that were males back in the past and they never did growl. My brother has a male rott and he does not growl when being petted. He is a little over a year old. It seems the only one I have noticed that growls have been females. I too get a little concern at times because she will get loud and at these times I will quick rubbing her to see what she will do and she will just lick our hands or face. This is how I know rather it is friendly or not.