What causes your elbow to lock up when carrying something an you have to pop it ?

This can be caused : by a loose fragment from an old injury. It gets caught in the joint, & your moving it about causes it to get out of the way. See an orthopedic surgeon & have it evaluated. Sometimes a loose fragment is made up of only cartilage, & will not be seen on x-rays, & a CT scan with some contrast needs to be done. Also an MRI can visualize them.