What Causes Crackling Sound In Knee While Squatting?

Whenever I squat (even bodyweight) I get this crackling sound in my left knee just about before I hit parallel, then it stops once I go below parallel. Once I begin to go back up, it crackles again during the same range of motion as when I was going down. It's not like the sound you make when you crack your knuckles, it's a crackling/crunching, not a popping/cracking. There is zero pain when it happens, and nothing feels weird, but it's definitely audible. I'm only 165 lbs so I doubt it's due to obesity or anything. It doesn't happen when just flexing my knee, only when I squat.