A mounth ago i fell down on my knee than had swollen and was hurting when i stood up.now i am hearing sound like 'krst' from my knee.is it meniskus?

Possibly: Meniscal tears are often traumatic , so it's possible that when you fell on your knee the meniscus tore. There could also be a different cause, such as patellar (kneecap) injury or bursitis.

Torn meniscus: It may be a torn meniscus (cartilage).It can only be diagnosed by doing MRI or a cat scan of the knee after examination by your doctor of an orthopedic doctor.So make an appointment and see what it is.