Yep-starter solenoid contacts. I'd be 100% positive of that. If you can pull the starter out and dismantle the solenoid (the smaller of the two cylindrical parts of the starter),you'll find that one of the contacts is worn away more than the other. There are two options-either file the other one down the same amount or go to an auto electtician and get some new ones (about $5-$10).
Don't fret, it's nothing serious and is cheap to fix. (I've re-built the solenoid on these starers multiple times-it's really easy)