Usually, a starter than spins but doesn't engage just makes the sound of an electric motor spinning rapidly..sort of a high pitched wirrrrr or whine. The starter solonoid ( an electromagnetic switch) is connected to your ignition switch and typically makes a "click" sound when engaged. It completes a circuit that sends 12v (and a lota amps) directly from the battery to the starter. The solonoid (on the ones in the old days at least) operates a movable rod on which is the gear that engages the flywheel. At the moment 12v is applied to the starter, the gear engages the flywheel and spins the engine. Sounds like you may have a sticking solonoid issue if it's making a rapid clicking sound but the starter isn't engaging. May want to make sure the contacts are clean.

sB