Why my knees crack every time i stand up from sitting ?

Knee crepitus: Occasional popping or cracking can be normal and is generally considered harmless- this is called crepitus. However, if the noise occurs regularly and is painful, it may indicate arthritis, chondromalacia, or patellar dysfunction . People who have regular, noticeable knee popping noises accompanied by pain and/or swelling are advised to seek a medical evaluation.