Actually Les, I'm not sure how much the pivetta (swazzle) is used in traditional Pulcinella shows, apart from those reinvented in the image of Punch. Remember that, unlike Punch, Pulcinella was more often seen as a live actor than as a puppet, and that the farces Pulcinella appeared in were dialogue rich. I understand that the live Pulcinella adopted a silly voice, but not using any instrument. The pivetta was used in the Burattini shows, but not all the time. It is quite obvious from some of the scripts that a pivetta could not have been used.
But then much the same has been said about the Payne Collier script and the swazzle.
To get an idea of some modern Pulcinella shows have a trawl through:
https://www.google.it/#q=pulcinella+burattini+videoI have not viewed them all, but a represenative number appear not to use the pivetta.
Descriptions of the pivetta sound much the same as the swazzle, although sometimes made of wood or bone rather than metal plates. I suppose the material used will affect the sound. But I think the main thing is the size of the plates, and the shape of the puppeteer's mouth.
However if, as I suspect, the Italian usage is for rhythmic accompaniment of slapstick sequences rather than for dialogue (a la Gatto), then it may well be made and tuned to deliberately give a shrill whistle rather than the more raspy sound that we prefer.
Actually, come to think of it, there is quite a wide variety of swazzle sounds despite the fact that most of the swazzles are made by one man. This suggests that it is more the shape of the mouth than the construction of the swazzle, which is the major factor.