She couldn't help wincing at the sound of nails scratching on a chalkboard.

wincing 是动词 wince 的现在分词形式。wince 的意思是"因疼痛、不适或厌恶而突然皱起脸或退缩"，常用来描述对刺耳声音或疼痛刺激的本能反应。