Sounds like the retinaculum that wraps around the knee. The Ilio-tibial band feeds fibers into that retinaculum, as do most of the muscles in the thigh. It can get tight just like any other structure in the body, and that tension can change as the muscles in the area and the fascial elements from the muscles that feed into it are doing work. Bits and pieces of the knee can get pulled a little out of place and "pop".
If it hurts: go to a doc and get a referral to a sports PT
If it doesn't hurt: look for a book on stretching for cyclists, or a good general stretching book (Jon Anderson's "Stretching" is a classic). Find an IT Band stretch that works for you. Add to that stretches for hamstrings, quads, calves, and hip adductors (the ol' "butterfly" stretch works great). Do a buttocks stretch, too. (knee to chest, knee to opposite side of chest) Stretch a couple times a day.
If that doesn't help things in a week or two, go to the doc and get a referral to a sports PT.
If your kneecap is being pulled out of alignment by something or one of your meniscuses is getting yanked by something, you really do want to get that taken care of. They can show you specific stretches for the muscle groups and for the retinaculum itself, and teach you to strengthen whatever might be weak. (usually in women it's the vastus medialis obliquus)
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