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My 2021 228i does this as well, but I am pretty sure it is a resonance issue coming from the underbody paneling as a result of braking in reverse. I am specifically looking at the panel just behind the rear axle line that is supported by lightweight metal framing, and thus more susceptible to resonance. Just assumptions at this point, but I plan to install some sound dampening material on the topside of those panels to see if that helps. This is why the sound tends to linger moments after you stop braking and why it can temporarily go away when you have your brakes serviced. IMO, it's a design flaw. I will keep you posted as to the results of the dampening experiment.
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