My long departed Goldie used to groan in ecstasy when you made a fuss of him. It didn't sound like his growl but it used to worry other people. He used to do it when he smelled a bitch in season too, even after he'd been neutered. He would lie next to where they'd wee'd and lick the grass and groan.
Old Dog makes a rhythmic huffing and closes her eyes when you pet her.
Yes!!! I was worried when he was a puppy that it was growling and used to tell him off but many years later I am now used to his very loud purring. Like a repeated contented groan really, on the outbreaths.