Any chance the starter is still being powered up after you've started, and ONLY in Park, not in neutral? I would have someone I trust to start the vehicle while you're laying under the truck and seeing if it's something you can easily hear and spot while you're down there. Starter motors that are made to speed with a " now-running" engine make some pretty interesting sounds since you normally don't hear them spin at such a fast RPM.
If it IS in the trans, you'll probably hear that as well while you're under the truck and someone is starting it for you. Let us know what you find.