I think maybe some of it is put on as not all women tennis players do it do they? Some you dont hear a squeak out of. Maybe it is to put the other player off, Or maybe it's the force of hitting the ball and it just happens, like the way you cant sneeze without making a sound. I've tried sneezing silently but it's really difficult. Womens tennis used to be really ladylike, they used to skip around the court all daintily and tap the ball all limp wristed. It's all very different now.