What could be the reason for getting a strange sound in your knee joints similar to the cracking of knuckles when you get up?

I'm not a doctor, but I have had this happen; it just means you're growing older. As you age--I'm as old as dirt--your joints will crack like this. If a doctor could answer this one more scientifically, have at it! If it isn't hurting yet, it will. It's early stage arthritis. You should see a doctor so that you get it under control before it gets bad. Some of the medications available can reduce the inflammation and slow the progression. It may just be the ligaments sliding over the bone ends. My daughter is just 17 and all her joints crack in this way, knees, ankles and toes and of course she can crack her fingers just by curling them up. No matter how quiet she tries to be we can hear her coming.