Why does the starter make a loud spinning noise and not crank over the engine?

If it happens intermittently, it is caused by a broken tooth or teeth on the engine ring gear. Thus when the starter solenoid throws the pinion gear backwards, the pinion and ring gear will not engage. In some rare cases the pinion gear can come off or the solenoid itself fails, yeilding the same result. You can test this by putting the car into gear and moving it slightly to move the ring gear (in some cars). Sometimes opening the drivers door and jumping up and down on the door sill can rock the engine enough to move the ring (again in some cars). The car might start. The only fix for this ultimately is to replace the ring gear.