What makes that crack sound when you open your tmj wide?

The disc: There is a cartilage disc in the joint that can get out of place. When you open your mouth, the disc may pop back into place and cause a cracking or popping noise. This is not necessarily bad, only if it limits function ro causes a lot of pain.

The Cartilage Disc: There is a disk between the bones of the temporal mandibular joint and your is likely malpositioned when you are closed, and it pops into place as you open. It is fairly common, sometimes causing pain and leading to TMJ dissorders.

Articular disc: That is the head of the condyle skipping over the meniscus (articular disc, cartilage) that is not in proper position.

The disc: The TMJ joint consists of 3 components. A fossa, a condyle, and a disc. The cracking sound is due to some combination of interference between those structures. If the cracking sound otherwise does not bother you - no treatment necessary. If it bothers you - see an oral surgeon.