possibly muscle tension in your gastrocnemius/soleus and medial and lateral muscles of the anterior side of your tibia. whole plant based foods will help with reducing inflammation as well as cardio vascular health, along with finding knots and strands of tension (that aren't ligaments or tendons) and massaging them out (milking them, so to speak).
My elbow was cracking during push-ups prior to eating fully raw.
It may be due to dehydration or something related to the diet since fruits are so hydrating and I no longer have this issue.
I think I remember from school that joins are filled with synovial fluid and if the quantity is insufficient, air is replacing it. The crack is generated by this air when compressed by physical activity.