My knee just keeps popping?

i injured my knee about a week and a half ago and i've been in a brace... now yesterday my knee was feeling really good and i was able to walk around on it without any pain...i had some popping and as the day went on it just kept getting worse... today the pain was back so i stayed off my feet the best i could(i have a three year old running around)...but every time i move my leg or walk my knee pops... it hurts most of the time, but it is very stiff.... it's not the ACL, as the doc has ruled that out, but said it could be the MCL... i'm not real sure about that either, as there was very little swelling at the time i hurt my knee... i didn't have any major sweeling until a week later... any ideas?

synovial fluid (fluid in the joints) is composed of water.

The knee is a joint that relies on ligaments and muscle/tendons to keep it together. In comparison, the hip is a ball and socket joint and has a bony configuration to increase stability. The knee is different and IF the MCL is involved then stability of the joint is compromised. Ligaments prevent excess bone movement and if its stretched it allows more motion between the bones in directions its not supposed to go.

Now don't get freaked out. Probably the ligament got slightly stretched and when you move the bones most likely goes in/out of place (just a little though) causing the popping. If the joint was really moving out of place there will be a whole lot of pain and lack of mobility (which is not your case)

Rest as best as you can, use heat to increase blood flow to the area (blood brings in nutrients / takes away waste products). If your on your feet and its irritated and painful then use ice to decrease inflammation. When your laying down move bend/straighten your knee in a painfree range frequently to minimize effects of stiffness.

the poping you hear is the releasing of co2 a gas that is produced when the body uses its white blood cells

you could have water on your knee.