Rubbing sound from Wheel/ Break?

[WoBoxster]

Off and on for the past month or so my back passanger side wheel/ brake has been making a rubbing sound. I didn't think much of it because it was a soft noise, you really had to listen for it. So today i was driving and it wasn't making any noise, then i pulled into my neighborhood and heard this terrible rubbing almost grinding noise from that wheel. I thought i had just ran something over and i had gotten stuck under my car. I got out and there was nothing there. I got back in and drove only a few feet and it made the noise again. I still don't know whats making the noise. It's comming from my back passagner wheel. I think it might be that the brake calliper is stuck, but when i drive it, it doenn't feel like the break is stuck. Any one know what this might be?