There are two situations you may want to look at, if you are hearing the starter spinning without the engine turning over, or the starter is making a grinding sound when it is spinning. If the starter gear assembly has a worn "over-running sprag" the starter will spin after the starter drive gear engages the torque converter Ring Gear. But, with worn Sprag Rollers in the starter drive the Armature Shaft will just spin with the Starter Gear will just sit there with maybe an occasional movement. The repair is to either disassemble the starter and replace the Starter Drive, or just replace the starter itself.
In the send case, once you remove the starter you can inspect the teeth of the torque converter's Ring Gear to see if there is any evidence of the teeth of the Ring Gear being damaged. Use a breaker bar with a socket on the engine's front vibration dampener to turn the engine during the Ring gear inspection to make certain you check the entire circumference of the Ring Gear. If there are damaged teeth, and you are getting grinding/tooth crashing sound when trying to start the engine, you will need to replace the torque converter, which will include a new Ring Gear. That entails pulling the transmission, so I really hope that is not your situation. That said, I have found more worn Starter Drive Sprag Rollers than damaged Ring Gears. In either case, if there is a problem it only gets worse if left unrepaired.
The prior posts in this thread offer some great suggestions also. And, many thanks to HemiKiller for the detailed 351C starter motor information. That is yet another tidbit he clarified that will make its way into my little library.