Thanks for your question! I am glad you are not in any pain. You may be experiencing "runner's knee" or patellofemoral syndrome, which is more prevalent in women. A creaking sound is caused by a misaligned kneecap grating your thigh bone.
Avoid high-impact exercise, which may be hard on your knees, such as running or tennis. Try low-impact exercises that will be kind to your knees, such as Pilates, walking and swimming. It may also help to buy new workout shoes that adequately support your joints.
Please visit your doctor if this continues to be an issue or concern. I hope that helps! Let us know how those tips work for you.