I'm just curious as to what a healthy amount of cracking is. People have told me their knees crack their whole lives and haven't had any problems. Mine crack a lot when I do ATG squats. It almost seems like a different type of cracking at the very bottom though. I don't want to call it grinding because it doesn't sound or feel like that, but it seems different then say cracking your knuckles. I have really tight hamstrings and have just started foam rolling. Would lack of flexibility be pulling on my knees with poor mechanics and should I just go until right before the cracking while I work on it?