a soft giggle
A soft giggle is when you laugh, but you're too shy to let it out. It’s like you're hiding your laugh in a pillow.
I did a soft giggle when I heard my mom’s joke. I didn’t want to laugh too loud.
He did a soft giggle when I told him my dog could talk. He was confused.
She did a soft giggle when I said I passed the test. She was trying to stay cool.