Are groans from Rottweilers normal?

My rott. is friendly, likes other dogs, good w/kids. I have one question. She moans when we pet her. Sometimes they are louder than others. She sounds like she is being grumpy, but when you stop she still wants you to pet her. I read in a book that rotts. do this and many mistake it for a growl. I just want to know if any other rott. owners have experienced this? I don't mind the soft groans, I just done like the loud ones. How are you supposed to be able to tell if your doing something they don't like, or they just really, really like it? I just think it's weird.