Obviously, take this with a pinch of salt:
You can research the topic more on the internet- Googling some of the key terms throws up hundreds of relevant links. If you are not comfortable with what you read on the internet, you should of course go and see a doctor. When I started front squatting, my knees started crackling and popping at the bottom of the squat quite remarkably.
Interestingly, a few weeks later I went home and had a two hour Thai massage. There was loads of work on my legs, especially ankles and shins. Next time front squatting, not a single click. Stopped doing front squats for a while, but I recently switched from parallel squats to full squats. First try, some clicking and popping at the bottom- noticeable, but not as much as when I started front squatting.
Second time, today, once or twice when I warmed up, but nothing thereafter. My knees used to make some snap-crackle-pop kinda like what you speak of. This was years ago and I\’d forgotten about it but just recalled it since you mentioned it. It also occurred during squats. In my case I now figure it was some sort of repetive injury stress causing it though I couldn\’t even begin to explain why.
Another factor for me was having a job which required my pushing heavy loads through casinos all day long to the tune of about 14 miles/day on carpeted concrete. At one point severe tendonitis developed and the only remedy I found was deep stretching. One technique I used was deep knee bends, sitting with my butt hunkered down to the ground almost, for about 15-20 minutes at a time.
Another one I found that worked was to sit with my feet and lower legs tucked under my rear with all my body weight over my feet. Soles of my feet pointed up, tops to the floor. Again, for 15-20 minutes at a time. Soon the tension in my knees resolved and I had no more pain or popping. (Message edited by fmn on August 13, 2006)