Knee pain is common in lifting, and it’s nearly 100% caused from a) poor mechanics and b) some sort of hip dysfunction. This Chalk Talk doesn’t get into lifting or movement mechanics, but instead addresses some palliative treatment and soft tissue work you can do to alleviate some of the pain.
Generally speaking, knee pain should tell you to work on hip mobility, especially with the rectus femoris. This muscle crosses both the hip and the knee, so if it’s tight, it’ll tug on it’s lower attachment near the knee (i.e. all of the quadriceps flow into a common tendon that attaches to the patella, which then attaches to the tibial tuberosity) and cause pain in the center of the knee, usually under the knee cap.
One thing we can do to try and alleviate tension is soft tissue work on the rectus femoris itself, and then on some of the other quad muscles.
Note that this precision soft tissue work works well when it occurs before stretching; it’s difficult to stretch a muscle that hasn’t had tension worked out of it. I like to follow this soft tissue work with the couch stretch, like in this video.