It sounds like she's just highly sensitive to sound. My guess is that she doesn't like the vacuum cleaner or the hair dryer either. ??? My son, when he was about this age and slightly older, ran every time I would turn on the hair dryer and the vacuum. Oh, and those hand dryers in some public bathrooms -- those used to scare him too. but just to the point of him covering his ears and running off -- sometimes he'd whine - but not cry. Screaming didn't throw him off though, and he was fine with lots of other loud noises too (applause, singing, train whistles). He grew out of it though at about 3-4 yr old. He's fine with any noise now.
I think at this stage you could try to distract her when the screaming starts -- just take her to the kitchen and give her some treats (have some special ones saved for this moment)... you need to reinforce that the screaming is fun - playful screaming - nothing to be scared of. So give her some special snacks then. Keep saying "it's ok. It's fun screaming." maybe practice some playful screams with her at home... and at that moment say "you try it. Mommy will yell - now it's your turn. silly!"
Just to help her get past this -- turn it into a fun game.
If it stays with her longer and becomes more serious - there is auditory therapy at some outpatient rehab clinics (Our Children's House at Baylor). They help children who are highly sensitive to loud noises/singing/applause, etc.