M.G.
I never had this problem with my kids - altho I've been around babies and toddlers that do it. So, I might be off base here but it seems to me whatever "punishment' you're doling out is not reaching her. If you are at home and she does it I would put her in her crib where she can't see and interact with you or anyone else. When she shrieks at wake-up tell her from another room to quiet and then don't go in until she does. Its more difficult out in public but if you're at the park and she starts you might have to just leave and take her home. Maybe this sounds harsh to you but from what you describe she seems to enjoy the attention and reaction she gets by shrieking - so you need to take that away. Good luck!