Here is why I'm asking, and this is some pretty good information I'll pass along as I had a similiar situation. I do alot of sitting daily at a desk, and as you get into your 30s and 40s, your legs will get really weak from lack of exercise. I think it is why my knees (both) started making a little popping sound which seemed similiar to yours, mine was right below the knee cap in front where I would get pain there taking stairs. The strange thing is I did workout nightly, but not really any leg workouts. I suspected that may be the problem so I started out by walking nightly for 30 minutes. I was even getting some mild knee popping then, but that was still mild compared to the knee popping when I went up stairs. I started by doing the walking 4 days a week, I couldnt do any leg weights because I was afraid to try it at that time. The amazing thing here was that after about 3 or 4 weeks of doing that, I noticed my knee popping was rarely occurring on the stairs! I was surprised as I wasnt sure that would help or not, but once I realized that I started doing light jogging the 2nd month with walking, and after a couple months my knees didn't pop again or hurt again on any stairs. I realized then I had to exercise the legs and today I do leg weights and fast walking.
By the way its been years since that was happening, I workout 4-5 days a week and my knees have never popped or hurt again since that time period. Its pretty crazy but it shows how bad all that sitting is for the legs. I think back to how bad it might be had I never discovered that and started doing that, hopefully this helps you past that.