Brake noise after drum to disc swap Options

[Syzygy]

I recently completed the swap from rear drum brakes to disc brakes. I successfully bled the brakes, and there are no leaks anywhere. When I first took the car out, I noticed a grinding noise coming from the rear brakes. Since I had removed any surface rust from the rotors, and check the pads for wear (they were in good shape), I assumed that the proportioning valve needed to be replaced, as it's different between the ST and GT, or so I was told at the dealer. So I found a convertible at a junker, got the valve, which looked to be in good shape, installed it, and..... no change. The noise isn't very loud, and it doesn't sound very messy - it sounds to me like the pads are riding on the rotors instead of gripping them. They're gripping enough to keep the rotors shiny, so....? I've tried anti-squeal stuff, shims, etc., to make sure I'm not getting any weird metal-on-metal resonances or anything. What could the problem be? Thanks all!

[Syzygy]

LewFX - was it constant, or only during braking? Also, what part of the shoes did you sand? I checked, and there aren't any obvious marks where things are rubbing.