Arthriis. Your mother most likely has arthritis. This can be confirmed with an x-ray.
What is rubbing knee. Need to know if there is swelling; when does it hurt - walking, up stairs, down stairs, sitting. The weird noise could be rubbing of cartilage, rubbing of bone, rubbing of tendons, shifting of fluid, joint mice, etc need more information.
Crepitus. She probably has arthritis possibly meniscal damage which can cause pain and clicking noises. Please have her a doc for xray, exam and possibly mri.