GM-GMC: Free spinning motor, auto parts stores, gm vehicles

Question
I have a 1986 3/4 ton chevy truck with a'7.4 Liter gasolene engine.
The other day I tryed to start my engine early in the AM and drained the battery. I then jumped the battery and after the connection was made I tryed to start the truck. I heard a POP and the whole electrical system shut down. I later checked the battery and discovered it completely drained. When I hooked up an alternate power supply to the battery I heard a motor running. When I removed the external power the motor stops.
I was wondering if this free spinning motor could be the clutch in the starter spinning and the without engaging? How can I test this?

Thank you for your help.
Kind Regards,
Ray

Answer
The motor that you are likely hearing is the heater and/or A/C blower motor running at low speed (in many GM vehicles, the blower does not turn off).

The only way to be certain about the starter is to remove it because the solenoid is on top of it and you cannot reach the wiring without removing the starter.

The starter can be tested for free by most auto parts stores.  You might want to get your alternator tested, too, as it may have been damaged when you tried to jump start the truck.