Greetings, friends.

Just to add my two cents worth to the mix. Without repeating the doggerel in the thread title, I would suggest it is a variant on the ancient art of "lilting" as practiced in Irish ceilidh when the participants were too poor to possess musical instruments. In order that the dancers could have music, one or more of the participants would "lilt" the melody. It's also referred to as "mouth music" and "puert a buill" (spelling questionable.)

In other words it's just a sound to go with the song.