Sound like the cog on the starter motor is not clearing the flywheel ring gear properly.
When you turn the key to start the car, a solenoid is energised which engages a cog on the starter with teeth on the flywheel. Once in this position, the solenoid hits an end switch which energises the motor itself which turns the engine over.
Once you stop turning the key, a return spring forces the now de energised solenoid back to its home position thus dis engaging the starter cog with the flywheel.
Sounds like in your case, either the return spring is week or the shaft that the starter cog travels on isnt clean and lubricated.
Not somthing i would worry too much about - may fix itself over time.
Cheers
Stewart - www.EveryCarPart.co.uk, December 2006
