Starter not disengaging

[Bubbalarge]

2001 Excursion 7.3. Started the truck today to move it quickly in the driveway and when I turned off the ignition the truck tried to start itself with a nosie like you would hear when you try to start the vehicle when its already running.
I could hear a fairly loud rattle from underneath and when I crawled under it sounded like it was coming from the starter. Tapped it with a hammer and the sound stopped and I was able to turn off the truck as normal.
Restarted the truck and had the same noise until a tapped the starter again.
Sounds like the starter gear is not disengaging after the motor starts?

Possible solenoid or complete starter replacement......how do I check?
Also, cost for new, OEM or aftermarket???

Thanks Bubbalarge