R.N.
My son does this too. It's a high pitched shriek too when he's frustrated and wants to say something but we aren't listening or understanding. It's hard in public b/c it's ear piercing. I attribute it to a lack of verbal skills. He gets so frustrated b/c he can't say what needs to be said. The only thing that's worked for me is to try to help him verbalize what he wants by saying I hear you you want x or i hear you, you're frustrated because... , etc. Sometimes, though, I can't identify the problem quickly-usually in oublic! : )
Good luck.
R.