If the brake pads and shoes are good, then it couldn't be metal to metal. My first guess is that the rain is causing a thin layer of rust to start on the brake rotors and drums. When you first start to use the brakes, the rust gets cleaned off. However, you still should check to make sure the brakes are in good condition.
As far as the noise, since it stops when you repack the rear wheel bearings, my guess is that the bearing is not being properly loaded (tightened).
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Wednesday, August 13th, 2014 AT 4:17 PM