Very similar thing happened to me. Knee locking and clicking pretty loud (and painful), and a feeling of tightness/pressure at the back which kept getting worse.There was a point whenever i bent the knee 100% of the time it would lock and click when trying to extend it. It went on for 6 months or so and the doctors were talking surgery and had various differing opinions of what the problem was.
It may be the same or it may not be, but in my case the problem was a misalignment of the patella and swelling of the adjacent soft tissues which turned into an egg and chicken thing with the swelling causing more misalignment, which caused more swelling, which caused more misalignment, etc
I'd recommend you visit a good knee physio. I tracked down and got seen by this guy who is meant to be one of the top specialists in knees
http://www.balancephysio.com/team/graham-anderson/In just a few minutes he assured me the doctors were all barking up the wrong tree and diagnosed my problem, which was quite the relief!
If you want to try what fixed me up, get a step in front of a mirror,and very slowly come off the step using the bad leg, like doing a one legged squat.
Look in the mirror to make sure the foot, knee and hip/thigh are perfectly vertical 1 on top of the other, and also when you look down the knee is right above your toes. Also make sure is smooth and there's no swaying or jerkiness. You want to be doing slowly and in control.
Repeat till you are comfortable with the muscle memory. Repeat also mounting the step. When you are confident you can do it right with no mirror go do your normal life WITHOUT using any lifts. Use only stairs and concentrate in going up/down slowly with the bad leg. If you don't come across stairs often then go find some.
It took me just a few days to feel improvement, and under 3 weeks till i was nearly problem free. Hope it works for you too