For those of us who have had ACL knee surgery we have all probably heard a painless cracking noise coming from our knee. This cracking noise can scare us because we don't want to go through that surgery and recovery time all over again. This cracking can happen at any time, you could be standing up or just simply extending our leg. 

There is nothing to worry about, I was at my doctors a little while ago and mentioned the cracking to him. After he looked at my knee and it cracked for him he came to the conclusion that there was just scar tissue breaking off, which is good because it will free up the knee to bend farther. 

My doctor also mentioned that if there is no pain nothing bad is happening.