What Is The Clicking Noise In My Knee?

My left knee has pressure that appears when I bend it with my body weight on it when it is ata 90 degree angle. I play volleyball and when I just if fees as if there is a void in the knee. It has began to click and feels like it is not supporting the movments and became weak o nthe landing. I am 35 in age 6' tall and 190 pounds. Untill now I thought I was in pretty good shape.