A popping sound in the knee may be a normal finding or it may be caused by a medical problem. In the normal joint, sometimes with rapid movement small bubbles form in the joint fluid and then 'pop' - this is called 'cavitation' and it leads to the cracking or popping noise. This kind of joint popping is totally normal and not a significant problem.
Importantly, it is asymptomatic, so if you have any other symptoms - such as joint pain
or swelling, or a catching sensation when you move the joint, then it is more likely that you have something going on and you will need to see your primary care doctor
for advice and evaluation.
One very common cause of knee pain together with joint 'popping' is something called patellofemoral pain syndrome, or 'runner's knee'. This is basically an overuse injury
in which the cartilage on the back of the knee cap becomes roughened and 'clicks' as it glides over the bone underneath. If you are diagnosed with this condition, it can be treated with rest, strengthening exercises, and anti inflammatory medications as directed by your doctor.
Make an appointment to be seen at your earliest convenience, and good luck!