I have an '87 Silverado V10 that is in super condition. However, while in motion, there is this scratching or light grinding noise that sounds to be coming from the brakes. It is not the pads, because I have visually checked them. Also, braking is fine... but that noise is kind of annoying while driving. I cannot hear it in the cab, just if I roll the window down and listen really good.

Anyone know what the problem could be? Rust on the rotors?

 

It could be rust on the rotors but that usually wears off quickly, the front brakes have wear indicators that are a little metal tab, when the pads wear down enough they start to make contact with the rotor making the noise you are hearing.