I was closing the pool last weekend and noticed these spheres rolling around in the pipe after I had turned off the pump. It took me a few minutes to figure it out but eventually I got it. There were air bubbles in the pipe that were trying to rise up the pipe but were being held down by the remaining siphon. As the struggle between the buoyancy and the siphon raged on, some of the bubbles would reach the surface while others danced up and down. When the bubbles had cleared, the siphon continued and caused an eddy current that distorted the way the pipe looked. Scientific Art??