What is the best exercise to strenghten knees?
Answers: Squats
squats or leg presses ride your bicycle (especially up hills)
do lunges
but you might want to wear a knees brace if they are giving you problems to help support and protect them while exercising
Squats are what my physical psychoanalyst has me doing. However, I lean against the wall and slide down the wall to do them. She have me doing them that way because they click and quality loose like you described. leg streches
You can't really strengthen your knees, unfortunately.
It's a combined, not a muscle. And every day, everything we do, our joint slowly ware down.
I suggest you do some stretches, or you could also try the suggestions everybody else made. You got nil to lose.
a sports trainer once told me that isolation exercises particularly the leg extension is terrifically good for strentghining the joint around the knee. hope this works for ya:)