With any luck, the rubbing metal on metal like grinding sound is just a small rock chip caught between a rear disc pad and a rotor. It may also be a rock chip caught inside one of the two rear parking brake drums.
If so, just be patient. Eventually, (usually a couple of days driving), the rock chip turns to dust with no damage done and all is well and quiet again.
As to the post re the intermittent error message, the good news is that it sounds like this is your first experience with trouble in the air suspension system.
I suggest that you review the files related to my air suspension albums per the link below to give you an idea of how the air system works.
The problem just may be a brake light switch or burned out or defective tail light bulb. Yes, read the files on the brake light switch as well. Just because you get a trouble message that infers the air system, do not assume that you actually have an air system problem.
The 3 is like a baby - very limited vocabulary - screams out anything when it hurts - it is up to the parent to figure out what the babbling means and if it is serious.
If you have the NAV with the 4x4 info display, turn that on and watch the vertical movement, or not, of the wheels. If one of them shows no sign of movement, say when you mount a curb, (one wheel at a time), then perhaps that is where the problem may be - and I say perhaps - nothing is easy with the air system until you first understand it, and then the problems become solvable.
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