Technically, the noise is very likely tiny steam bubbles that collapse with great force as the steam condenses, thereby shrinking to 1/1600 of its volume. This is called cavitation and the forces appearing at the microscopic scale are enormous and present a significant challenge in a number of engineering tasks. For more details, see the Wikipedia article.
Now, with regard to the latte, I'm just a latte consumer, and don't really know anything about how to make it, but the above might still be helpful in connection with what others have written. I would expect cavitation to occur every time, and in my experience, there's always some kind of noise associated with the steaming. However, that high-pitch screaming might mean there's more cavitation, for instance, because the steam is hotter, the milk is colder, or there is more steam etc. Each of which may very well affect the result. Since milk is quite a complicated emulsion, I'm certain with those tiny implosions, there's a lot going on at the microscopic level.