What Causes Popping In The Knee?

I have 36 years old and a very active fitness instructor. I have been through PT (ASTYM) for patellar tendonosis. I was having relieve and not that I full resumed high impact activities, I hear a very distinct popping on that same knee. There is no pain at rest and a very, very, mild tenderness if I overdo it working. However, there is popping every time I straighten my leg or walk down stairs. I am very frustrated because I finally got rid of the pain of months of therapy but now this popping? I am doing more damage? Please advise. Thanks!