Why might my knees make a crunching noise frequently?

Cartilage damage: Knees can crunch for many reasons. Common causes include varying degrees of cartilage or meniscal damage, loose bodies, and scar tissue. Osteoarthritis is the term we use for advanced cartilage damage.

Patella-femoral join: Usually the crunching you feel especially when you get up out of a chair comes from some break down in the cartilage surface on the back side of your knee cap. As you apply a load to the joint, the noise becomes apparent.