My Right Knee Keeps 'popping' More Now Than Before, Especially

My right knee keeps 'popping' more now than before, especially when I move laterally--either to left or right. It tends to 'pop' less if my knees are slightly bent. The 'popping' noise sometimes are very loud and accompanied with some pain. Once it starts popping that day with movement, then it tends to pop more often after that on the same day. What is the problem--ligament? I'm a 48 yo male. I did play a lot of soccer when I was young. And I retired from the Army a year ago. Thanks. Tom D.