Toddler with high pitched scream

My daughter is 18 months. After about 10 months, she began screaming. Not crying. High pitched, ear piercing screams. It's been 8 months and it hasn't stopped. This isn't the "she's discovering her voice" anymore. She does this all day everyday. She doesn't even cry anymore. She just screams like that.

It's too the point where I cannot get errands done. I can't take her to the store, restaurant, anywhere in public. I can't even stand to have her in the car with me anymore because the ENTIRE time is just high pitched screams. I have a headache all day everyday, it's killing me!!

I have tried teaching her to not do it, I've given her toys, music, tv, binky, food, everything to occupy her. It doesn't work. I don't know what else to do. I have considered wearing ear plugs it's that bad and that loud.