@MikeMason-- I'm not a doctor but if you experience this all the time, it might be a sign of weak knee muscles.
I used to experience the same before I started working out regularly at the gym. It's definitely not due to injury but due to insufficient use of muscles in the knee. Once I started to do knee exercises and strengthen the muscles there, the popping sound disappeared.
If you start having knee popping with pain though, you should see an orthopedic to figure out what's going on.