I had acl surgery may 15, 2014 and for about a 3 weeks twice a week i would kick a bag n do mits light, i hear a cracking sound when i flex my knee?

Likely ligaments: The knee joint has several bursa, which are areas where the ends of ligaments are allowed to move in and out of the bursa when you are walking or exercising. The bursa contain fluid that allow for smooth movements and the sound you here may be either a lack of bursar fluid, or decreased synovial fluid in the joint itself. In the absence of pain this is not a serious concern