From what I've seen, dogs have variety in their growls just as they do in their barks. They growl when they play, but growling can also be a precursor to a bite. Dr. Ian Dunbar (who I would consider an expert) puts the growl at a 'Level 1' on his Bite Assessment Scale - where the bite hasn't actually happened yet - it's basically just intimidation, along with snarling, lunging, air snapping.
A very low pitched growl, I take in doggie language to mean, "C'mon. Make my day" lol. It's a warning but you have to ask for the bite. Then the nasally growl, which is more panicy and fearful, is more of a promise that you'll get bitten. This is what I've observed anyway.