You might have a bad caliper or calipers. I just recently experienced something like this. On my Merkur I heard the squeak and checked a pad. It was fine. I checked the right rear pad and it had plenty of wear left. I just ignored it because my brakes are screwy anyway. Then I heard this awful grinding noise...even if I didn't have the brake on. So I took it in. It was my Left rear (remember I checked the right assuming that they would wear evenly). What happened is that my left rear caliper was dead. So it was constantly applying pressure to the rotor. So you might have something like that going on. I would get it checked out. If you have a bad caliper they need to replace it. Otherwise, just get the pads...it will save you from a headache. Good news for me that the rotors could still be turned since they were new.
