Perhaps its my poor explanation.
In the past as soon as the engine fires the Starter Motor is disengaged and (I assume) the Solenoid cuts power to the circuit once the key is released from the "start" position.
Now for some reason when the engine starts the Starter Motor can be heard whirring away for a second or two as the key is released. I don't think the gears are still meshed otherwise I would have expected more of a graunching sound. This just sounds like an electric motor running freely.