
Hi,
I'm not 100% certain, but it sounds like it could be the same kind of noise that my boy makes when he's pleased about something, he makes a kind of growly/grunting noise. Again, no teeth showing and his tail is wagging. Your friends dog could be doing the same, some dogs are just quite talkative.
Mine has always done this though, and if your friends dog has only recently started this it might be worth checking that he's not in any discomfort, as it could be a quiet 'I don't really like that' growl that he's doing instead.
Claire :)