It's normal in young macaws, in older macaws it could indicate a diet related disorder. Here is the medical description:
Articulations are divided into synarthroses, in which the osseous components are united by fibrous tissue or cartilage and diarthroses. Diarthroses are frequently involved in pathologic changes involving any of their anatomic structures: the fibrous joint capsule (where the sound comes from,) synovial membrane, hyaline articular cartilage, subchondral bone, and intra-articular ligaments (and also the menisci in the joints). Joint sounds may be caused by exertion and luxation.
Normal synovial fluid lubricates the synovial tissues in a joint through boundary lubrication, including a glycoprotein expressed from the cartilage during weight bearing. The synovial fluid also nourishes the articular cartilage. Any joint exercise alters the volume and composition of the synovial fluid (as a result of an increased permeability of the synovial membrane relative to blood components) and increases the intraosseous pressure in the involved bones.
In other words while your baby is growing this excursion (or exercise) is strengthening their muscles and will allow these popping sounds to go away with time and proper nourishment. This is a normal sound while they are still juveniles. If this continues into adult a nutritional re-examination will have to be performed.
