It is a common problem, he explained. Lightweight starters use fast-spinning permanent-magnet motors and steep gearing to achieve high torque. Some models, when they are not energized, cannot be turned by hand. The starter engagement mechanism in many Continental Motors engines, including my TSIO-360, consists of a coil spring that tightens around a shaft. When the starter stops turning, it's expected to freewheel, and the spring is supposed to relax and release its grip. It's an ingenious system, but if the starter doesn't freewheel, it keeps the spring rubbing on the spinning shaft, eventually wearing both until the spring can no longer grip the shaft tightly enough to turn the motor over at all. Hence, the whirring sound and the stationary propeller.