I had hard fall on school playground when I was in 8th class which lead to pain and I couldn't run for a month. I didn't go to the doctor that time as it didn't look any serious. That knee injury had given me popping sound in my left knee whenever I tried to bend it with speed slower than normal or fast speed. Its been 20 yrs living with the issue. I have no swelling but sometimes I feel pain after I use my leg / knee more than usual like in running, squatting. What could be the reason behind the popping in left knee only.