A few days ago my truck started making a noticeable sound from the rear half of the vehicle. It sounds like a brake rubbing, but I checked my brakes and everything is fine. It makes the noise from 10 mph and beyond. I've read some posts on the board, but nothing describes the exact sound I am hearing. No high pitch squeals or ticks, just a rubbing sound. I also checked my brake dust 'disc' and it wasn't rubbing.

Any ideas?

2001 F150
3.55 LS
5.4L automatic