"Warning growl" refers to the low sound made by an animal, usually a dog, to show that it is upset or trying to alert others of a potential threat. Full definition
He's never ever shown any aggression to other dogs other than the odd warning growl if they do try to go for him and that's usually only if he's on a lead and if he feels really threatened. (leerburg.com)
They may leave their small children alone with the dog, only to be surprised when the Chi bites them, after many warning growls communicating that it did not want to be hugged and squeezed like a teddy bear. (littledogtips.com)
When a dog rumbles, it sounds more like a boat motor (even pitch sounds, almost monotone) than a deep throaty, fluctuating warning growl. (dogtrainingnation.com)