Crackling if occurring on its own is normal but u should not do it yourself as it lenghthen ligaments which allows excessive movement at joint and can cause arthritis to occur early. The ligaments take more than a week to come back to normal lenghthen. Break the habit.

Hello, cracking of knuckles is very common. It involves that you bend your joint  either ways in order for it to crack. If it becomes a habit then you will try to crack it often and may cause excessive load/pressure on the joint and this might cause damage to your ligaments and capsule around the joint. So avoid cracking of knuckles for few months and see .Don't worry its not harmful