#1-do you have full knee extension?  I’m assuming that’s a no based on what i read.

#2-I’m also assuming by “PT” you mean physical therapist, and if so, did he/she do a full eval to check ROM of the hip/thigh/lower leg/ankle/foot? That would help key in with where to start.

#3-Does said “pop” hurt?

Pain being the symptom, unless you abruptly did something traumatic like a knee sprain during running or jumping (at any point), I would say its probably assocaited with some sort of dysfunction up or down the chain. Address ROM of the muscles surrounding hip/knee/ankle/foot. Don’t torque on the knee joint, like i said, chances are its due to lack of ROM of the surrounding musculature.

The two big ones i would see about:
Your hamstring flexibility on each leg separately (where you’re unable to keep a flat back).
You hip flexibility, predominantly the rectus femoris.

I used to have a “click” near full knee extension (not painful), which could also be called a “pop”, following my most recent ACL surgery. I found addressing hip weakness and mobility in the musculature surrounding the knee and hip to be most effective in eliminating this and restoring proper ROM to my knee joint.