My DD did this aswell. Actually she was around the same age as your DS when she started. It lasted 3 weeks or so and then started up again when she was older (maybe around 14mths) I don't know why she did it and I didn't like the sound of it either.

The only thing i could think of was that maybe she was teething. Because around both those times she did get more teeth. Other then that, she might have accidently made the noise and then liked it...

I am not sure but I hear it is fairly common and alot of babies/toddlers do it. They do grow out of it