Pop behind knee feels like acl rather than scar tissue

So I’m 5months post op and everytime I go from 90 degrees to lifting it horizontally 180 degrees, I get a pop (feels like something dropping into a hole), pretty much as soon as I extend. Every time.

I’ve been working hard to build strength but this has been on my mind. Could this be the ACL? It’s directly in the middle maybe meniscus?