It's the belt squeaking on a pulley. When you shut off the motor it might rock back and forth for a quick second and the belt will slip on one of the pulleys and make a nice squaking noise. Mine was accompanied by constant squeaking when running. The only way I got rid of that annoying noise was by using a gatorback belt from goodyear. Tried all brands of different belts, that's the only one that won't squeak.