What Causes Pain In Knee Pain?

i always had a sound when i bend me knees, i used to play badminton and other sports as well. from past two months i am having pain in my knees. it started from right knee and then left and now in both the kneed. i was doctors and they give me MR ( Muscle relaxcent) tablets. Later I saw ortho and he says its nothing serious I need to take rest. Also he recommended me celadrin for 2 month.

Can u just assist me is access badmiton playing can lead to this problem or shall I go for MRI,
When I take rest I feel little better but the moment I take stair.