Pain: Snap, Crackle, Pop

There are several reasons for noises in a joint most of which are nothing to worry about

1. Cavitation. Literally forming a cavity in the joint fluid. This is a popping sound, most often painless, which occurs when you move a joint. It can occur it many different joints but cracking knuckles, knees and necks are the commonest. A bubble forms because the pressure in the joint changes. All joints have a small amount of fluid in them (synovial fluid) and all fluids have air dissolved in them. When the pressure is changed the air comes out of solution and forms a bubble. We see this most commonly when we open a pop bottle, but you also see it when you row a boat - bubbles form beside the oar blade when you pull vigorously. Cavitation is commoner after injuries because there is often a bit more fluid in the joint as a result of the injury. However, in itself it is harmless.
2. Scarring in a joint. If you bleed into a joint the blood will coagulate and form a strand of blood clot inside the joint. This blood clot will transform into scar by the process of healing we have discussed elsewhere. So now you have a strand of scar tissue crossing from one part of the joint to another. If this catches as the joint moves you may feel it as a snap or a crunch. It may be slightly painful. Normally the scar tissue will stretch and thin out in time. This is often only a minor problem and rarely needs treatment.
3. Scarring of the joint lining. In the knee there is a fold of the joint lining which is usually quite small and pliable. It stays out of the way. However, if you injure your knee the bruising may collect in the loosest part of the tissue, in this case the fold, and make it swell up. The swelling is transformed into scar tissue and the fold is made thicker and stiffer. If this happens it may start catching in the knee joint. It is difficult to diagnose this and make sure about it; it comes on long after the main injury when you are getting back to normal. However, it is easy to treat. The fold serves no purpose so can be cut away without any long term adverse consequences. This is called a partial synovectomy as the lining of the joint is called the synovium and an "ectomy" is the removal of something - synov - ectomy = joint lining removal.
4. Damage to the joint surface. If the fracture line entered the joint the smooth joint surface will have a healing line (scar) in it. You may even have steplike irregularity of the joint surface if it proved impossible to make it smooth and fix the intra-articular fracture exactly in position. This would be one of the more serious reasons for having grinding or crunching in your joint as the potential for post traumatic osteoarthritis would be increased.
5. Cartilage tears. In the knee there are two gristly crescent shaped structures that sit between the thigh bone and the shin bone. The medial one is on the inside of the knee and the lateral one on the outer side. They are often torn in sports injuries where the knee is twisted. However, they can be torn when the leg is broken and this injury will not become apparant until the fracture has healed and the knee is being used more normally. Persisting painful clunking, locking, collapsing or giving way of the knee are the hallmarks of a torn cartilage. Unfortunately the cartilage doesn't heal because it doesn't have a blood supply.
6. Ligament Injuries These injuries most commonly show up as a feeling of painful instability. The joint moves funny or slips. However, this feeling can be interpreted as a pop or a crunch so I have included the possibility here for completeness.