My Knees Have Started To "click" Occasionally When A Bend

My knees have started to "click" occasionally when a bend over to tie my shoes or when I do "lunges" at my senior aerobics class at our local YMCA. It seem "positional". I have occasional discomfort, but nothing chronic. I can do stairs fairly easily, but again, have occasional discomfort. I am otherwise healthy, but have a concern regarding the right kind of exercise. Should I have an evaluation?