The Ready State Virtual Mobility Coach is like having a virtual Kelly Starrett in your pocket.
Does it only occur when stretching the hamstring in a fully extended range?
Is this a new occurrence?
Could be the IT Band rubbing against the side of the knee.
A couple episodes to check out;
Pro Episode # 35 – MWod Pro-User Request Friday: The IT Band Primer
Episode 142: Tight IT Band and Hip Flexor Fix: Runners?