A.F.
Well I don't have any advice but I know exactly what you mean! My 18 month old does the same thing. It's usually when he is excited. It's ear piercing!! His favorite book is Goodnight Moon and b/c of the little old lady whispering hush...he has learned to put his finger to his lips and say...shhh. So sometimes that helps. Otherwise...the only advice I can offer is to give her something else to do. My son LOVES silly noises so I will make a quiet noise and he will copy it. Many times he is happy to make the new noise and stops squealing for a while.
Good luck and remember...at least we have happy kids!