[singpilot]

On top of the starter is the solenoid that drives the pinion into the ring gear on the crank.

It sounds like it has failed. If you are good at that kind of thing, you CAN change just the solenoid. You will have to find a specialty starter shop to get the parts, most places will carry only the complete starter.

If you wanted to verify prior to pulling it, take the wooden handle of a hammer and 'malletize' the starter, then crank. Beware the wires around the starter, they are fragile and hi-amperage. It may work again for a while, but is telling you that it is failing.

Let us know how it turns out.