What Causes Squishing Noise In Knee Joint While Walking?

Hi. I hear a squishing noise in my left knee joint when I walk upstairs. Also after walking down 2 mountains I have pain in both knees. This pain subsided after a few days. However, a few weeks later whilst standing still I got a very sharp pain through my left knee cap which went after a minute or so. I now have pain in my knees when I walk down hills or downstairs.