Probably a sticking solenoid, the one that piggy backs the starter motor. The armature corrodes and this inhibits movement within tight clearance housing. A tap with a mallet or block of wood (not a hammer) is a temporary cure in an emergency, best performed whilst someone is turning start key simulataneously. Permanent fix is to replace solenoid, this may involve some heavy duty soldering though. You could also try getting some releasing fluid inside.
