Popping or crunching sound in
knee can be due
to arthritis (it can be rheumatoid arthritis too). It can also be due
to medial or lateral meniscus injury or patellar injury (caused by active sports, dancing, exercising or accidental). Another possibility is Chondromalacia patella in which the knee cap keeps rubbing against the underlying cartilage of femur (corrected by braces, taping, non-steroidal anti-inflammatory, physiotherapy or surgery).