I don't have a rottie exactly, but Leo has part Rottie in him. He doesn't growl when I am petting him, but be does make some funny gurgling sounds. I do deal with quite a few Rotties at the shelter I volunteer at and they are all general love bugs. They love to lean into you to get closer (Leo does this too). Vocalization does not always mean she is actually growling, although it might sound like it - I am assuming this is what she is doing.

If she is licking the muzzles of other dogs, that is actually submissive appeasing behavior. She is probably doing it to show them that she is no threat.

She sounds like a sweetie - hope you post some pictures of her soon.