Why does it sound like my jaw is breaking when I bite down on an old crown? Not loose/feels ok.

Hard to say. It could be a problem with the crown, or the substructure of the crown, or the tooth itself. Or it could be a grinding sound coming from the joint (tmj). Either way you should see your dentist to evaluate, determine the cause, and discuss your options.
Jaw or tooth problem. It could actually be your jaw joint or a cracked part of the tooth in a spot you can't feel. I would recommend having an exam just in case there is a significant problem.
Loose. It may not be loose on the surface but a crown goes deeper which may be cracked and not visible.