I think my starter is starting to give me trouble. Lately when I try to start my 1984 F-100 it makes an awful sound. The sound is hard to describe. It sounds like the starter stays in engaged for a while and continues to spin.

 

Kind of like a grinding noise huh? Make sure you aren't turning the key too far foreward, my 82 F-150 does this if i turn the key too far foreward, the return on the ignition is weak, so i have to pull the key backwards to keep it from doing this. also id check your solenoid and your starter cable going down to the starter, it might have a small bare spot and ground to the frame once you apply power to it.

hope this helps!