K.S.
My niece did this at about the same age, and my sister-in-law came up with a brilliant approach she called the silent scream. Whenever her daughter did the loud scream she would exaggeratedly pretend to scream along with her, but with no noise, holding her hands out, rolling her eyes, the whole thing, and then by just putting her hand to her lips, not saying shush, but acting it out, her daughter eventually got the idea to make no sound at all. Just acting out the scream without sound became the fun part and had the pleasant outcome of being able to reduce a room full of adults to belly laughs when her mom would say, Come on, Mady, let's silent scream for them!