seahorse

rear brake noise when backing up, best described as dying seal. not a grinding sound, rubbing, scrapping,squeal, but a LOUD howling noise. Noise is random, not due to wet brakes, rust, ETC, but only when backing up. Noise started after they did a new brake job. Been to dealership 6 times they are unable to do anything about it. No service bulletin available. anyone else have this problem? Had video but too large to post.