Howl, from a bad drum face ?? Rear drum brakes generally self adjust when brakes are applied in reverse. Try applying the brakes harder in reverse a few times to help verify brake adjustment rather than softly applying them all the time. Perhaps there is some corrosion on the drum face, brake shoe is wearing unevenly ? Have you taken the tires off and looked at the inside of drum and brake shoe wear ? Most dealers will perform a free brake inspection, that way you will know what the issue is.