I can do what OP was describing on inhale and exhale, and I have a small amount of didgeridoo experience, but I don't think circular breathing works here. Mind you, I'm not GOOD at circular breathing, but my understanding is that it works by using the cheeks to force air through the lips as you are inhaling.

The purring sound here though comes from too far back to make that work, I can't get air to pass through that part.