Another possibility is the knee cap
rubbing against the underlying cartilage of femur. Having said this, at times the noise
has no pathology behind it.
The various causes of knee locking are torn medial meniscus, mis-aligned knee cap, osteoarthritis, knee arthritis, torn lateral meniscus, osteochondritis dissecans, recurrent patella dislocation, chondromalacia patella, knee alignment problem, knee fragment or knee cartilage disorder.