What other than a blown rear wheel bearing could make a similar noise?

On an old rear drive Volvo 940 with disc brakes, there is this constant grinding noise. However, it's accompanied by an odd sound from the rear brakes when they're applied. The wheels are solid and rotation isn't impeded at all. However, on one brake disc, there is a distinctly odd wear pattern with a deeper groove than on the others with what looks like a strange line on the middle of the disc. There looks to be plenty of pad left. Any ideas?