While the older ones do scream and are loud a lot I distinctly remember the screeching phase for each of my kids. It was the worst in the car due to the enclosed space. Maggie was much worse than Molly (molly had Maggie to distract her in the back seat). I just put up with it and tried to distract with some music and DH would open a window to kind of blast with some air (she actually liked it and it was another way to distract). For us it was like they figured out they could make this super loud noise so they did it over and over and over. I feel for you, it is not one of the fun phases.
Yup, DS "discovered his voice" around the same time and went through a couple of weeks of shrieking for the joy of it. We tried to ignore it, and now it's just occasional.
Re: random screaming/screeching?
DD is 22mo old & loves this. She'll often preface the screaming by saying "no screaming"