The puzzling 'happy noise'

Both my children have something they refer to as their "happy noises". This noise of their sounds suspiciously like whining. I honestly can only tell the difference if I can see the look on their face. I can't figure out where they got the idea that this whiny, moaning, chirping thing they do has anything to do with the expression of joy....

Sometimes they copy each other so much that they create a norm with something that might be a social taboo.

I have thought a lot about this 'happy noise'. Where did it come from?
After serious thought, I think it comes from our dog (who btw cries more like a bird then a dog). I think they might think his crying is his 'happy noise'.

I really can't be sure what those little crazies are ever thinking though.