What Causes Grinding Sensation In The Knees Upon Bending The Leg?

Hi I started suffering a weird sensation in my right knee, more specifically around the kneecap? not entirely sure. Everytime when i stand up and bend my knees I feel something, maybe some grinding or poping in the knees. sometime It hurts a little bit, other time it feels like nothing. I can walk fine, and if i sit down or dont do activities i dont feel pain either. Only when i stretch it i feel a weird feeling and hurts a little bit sometimes. Also when I put my hand down near my kneecap, maybe anatomically near the patellar tendon. I feel some stress and pain, I do not feel pain when pushing or putting some force on the same place on my other knee.