Piping sound -define. Piping - what does it sound like? If the knee rubs it could be bone sliding on bone like the kneecap slipping and grinding. It could be tendons rubbing across surfaces. Fluid can pop also also. Is it both knees or just one? Can you tell which side it is on. Inside or outside. We need a definition of piping.
Have it evaluated. Sound coming from the knee could be due to tendon, cartilage, or bone abnormalities. Have it evaluated by your orthopedist and find out why it is making that sound.