Has she always been sensitive to noise/sounds?
The reason I ask is, my daughter has always been "noise sensitive" since she was a infant. Some babies/children are like that. Perhaps, it's noticeable now, because she can talk and say it.
My girl, also did not like loud cars, or yard work activities, buses, trucks, amusement parks, ANYTHING that was loud. She even couldn't sit through a Sesame Street stage show because she said "it's too loud."
Or, it could be car specific. But no worries, kids at this age, and at other phases, can and do have certain "aversions" or fears...it's a changing cognitive development...they also start to have night-mares etc.
Or did something happen to her, that was car related?
Don't force her "acceptance" of it though...I"m sure it is a phase that will pass. But, if she is indeed "noise sensitive"....it will take time. My girl is still sensitive to noise/sounds...even the school cafeteria is "too loud" for her...but now that she is older, she can adjust to it more maturely.
Just comfort her... show her that you "understand" etc. Let her lead you in how you should pace her and yourself in certain environments. My girl, when she was that age, didn't even like Chuck E Cheese... because it was "too loud" and it was just not tolerable for her.
Well if anything, perhaps speak with your Pediatrician? All the best, hope this helped,
~Susan