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Clinical examination followed by MRI of that knee would be required to proceed further.

Its perfectly normal. Don't worry as you are only 20 yrs old... As long as there is no pain in the knee and you are able to do all your activities, there is nothing to be done. Need to intervene only if the "sensation of grating" of the knee is present all the time upon extension, even when sitting with your legs hanging from a chair / bed. Or if there is pain. That sound upon squatting (Hyper flexion) is just cracking of your knee just like the cracking of your knuckles.

May be meniscal trap during squat Visit doctor for examination of knee