What Causes Popping Noise In Knees After Undergoing Arthroscopic Surgery?

when I was 35 i have othoscopic surgery on both my knees the doctor scraped the back of both knees and fixed a cartilige tear in the right knee. Anyway, my knees have been feeling good for the last few years, I notices about a week ago they started cruching again so I thinking its time to see the doctor. Last week the left one started making a popping noice when I walk up stairs, and stand up. I workout and take care of my self...what can be causing this popping noice it does not hurt.

thanks,