I'm waiting for my Dad to e-mail with the "official" explanation of why Puerto Ricans have this custom. There was a song he was trying to remember that they sing when the plane lands. I'm already in trouble for calling the cuatro a 10 stringed guitar when it is a 10 stringed instrument. (I just try to print the facts, not argue semantics.) I do have the name now of the scratchy gourd thing. It is called a güiro, and it is traditionally played with the cuatro.