Originally Posted by dbarnes
This is a common noise and not an issue/problem of any kind with your car. The M3 uses drum brakes for the parking brake, not the rear brake calipers/rotors. To prevent corrosion/rust build up on the drum and shoe pad material surfaces you need to regularly clean the surfaces by applying the handbrake (lightly) while driving.
If you want to test this the next time you park, on a flat surface, do not apply the handbrake and then see if your car makes the noise when you "stand up/sit down".
So what you are suggesting is that everyone else on the forum has at some point, or could be experiencing the same issue.
Not doubting you or anything, but I find it difficult to understand why BMW would use such a low end technology for the e-brake, when my previous Nissan used disc brake for this purpose.