Why does my cat hiss whenever she's kneading a pillow or blanket?

She's maybe almost a year old now and she's been doing this for the longest time. Whenever she'll be kneading her paws into a blanket, pillow, or any other... soft material - she'll start to hiss very loudly. She's not in pain at all and there is nothing around to peeve her or make her upset. Anybody got any thoughts on this?