Are we sure this is a growl? Some dogs, rotties in general, and my little beagle mix, "grumble" when happy. It does sound like growling, but the body language is very loose and relaxed. The first time Kabota did it, I was petting him. I immediately stopped petting him, figuring I had hit a sore spot, and he wiggled even closer to me. Grumbling is just how he lets you know he's happy.