What causes a popping sensation on the left side of my chest and sternum? NO PAIN. Almost feels like when you crack your neck. PCP says heart ok.

Costal cartilage . The most common cause of the popping sensation you describe is separation of the cartilage on the ends of the ribs from the sternum. There are many reasons this can happen but the important fact is that it has no medical significance. occasionally this condition can be accompanied by pain which is best treated with NSAIDs like Ibuprofen. In the absence of pain there is no need to worry. .
If your heart is ok. it is probably something on the chest wall. Sometimes, you can get inflammation of the area between the cartilage and bone of ribs. It should heal with time, If pain persists, see a surgeon to make sure there is nothing wrong with your breast or blood vessels of the area.