First and foremost: Get to a doctor.
It may be nothing serious; I asked my doctor about a similar sound in my knee, and he assured me it was nothing to worry about, it's a common side effect of aging and has to do with reduced cartilage.
However, that was my doctor, after a thorough physical exam, and listening to the sound my knee made. Yours could be caused by something different. Stop the exercise, see a doc, and make sure everything's okay. Better safe than sorry!