I recently purchased a bike that clearly needed fixing, for a great price. I've given the bike a bit of a tune-up, but one thing that won't seem to go away is the squealing sound coming from the disc brakes. The brake pads look fine, but I went ahead and cleaned them with isopropyl alcohol and shaved them a little with light sandpaper. I also cleaned the disc with the same alcohol, but the sound is still there. The disc itself appears to be straight. Any other thoughts?