That's the sound Nathan is making as he blows bubbles. He's actually managed to figure out the bubble wand, how to stick it into the jar, and he can blow bubbles. I mean, most of the time he's just making a mess and/or ingesting trace amounts of bubble solution, but it's so goddamn cute.
Of course, right after that he spilled the entire bottle on the coffee table and floor. Hey, at least it's just soap!