I'm 28 years old, but my knees have started making these crackling sounds. What is it? There isn't any real pain involved, but they make this horrible noise when I work out and when I climb flights of stairs.

Cartilage wear. Cartilage, which covers the bones, is smooth. This smooth surface along with joint fluid, help to maintain painless joint motion as a result of normal wear, cartilage develops surface irregularities which create a grinding sound when making contact. In the absence of pain continue to strengthen the muscles around your knee and moderate the amount of stair climbing and plyometrics.
This . This is referred to as "crepitus". As long as there is no pain, there is really nothing specific to be done about it. If there is associated pain, it could represent inflammation, torn or uneven cartilage, or other issues that should be evaluated by a doctor.