Back of knee 'clicking' when jogging/ running

FTIM2015

Ok, so I stretch before I start but after a few minutes of walk - jog - walk - run the back of my left knee starts to 'click' almost like the tendons are catching on each other?!

Any suggestions of how to stop it, strengthen it? It's not painful but it's unconfortable. My dr's have never been sure whats wrong with my knees, nothing shows up in scans and physio have given up with me!