the starter motor drive contains a one way bearing.it locks up while starting but as the engine speeds up and spins faster than the starter it free spins and can make those sorts of noises as clutch dust can get into them, they are a replaceable part for around 30-50 but once you factor in labour and the condition of the rest of the starter your prob worth pricing up a brand new one.
taking it out and tuning the gear by hand it will probably be rough or notchy in one direction