Knee making a cracking sound!! Is it because of poor posture??

I am quite thin, so I am more worried about osteoporosis and I take milk and other calcium-rich foods every day. Every time I stand up, I hear a cracking sound from my knees. It's embarrassing, and I also worry if there is something wrong with my bones. I was told not to keep my knees bent for a long time, so I try to keep my legs straight, but it's not easy. Before I realize it, I find myself bending my knees again. Once, perhaps because I sat cross-legged for a long time, one of my knees kept hurting. Since then, I have consciously kept that leg straight, and it has gotten better. However, the cracking sound from my knees still persists. It seems my bones have become quite weak. Should I take calcium supplements separately? Is this a warning sign for my bone health?