I feel for you man....it happened to me at low parking lot speeds...embarrasing. Knock on wood it went away and has been gone for quite a while. Damn, I just jinxed myself.
Anyways what worked for me was gently pulling up the parking brake slightly a few times in a row while creeping along.
Don't worry, this sound is included in the new M4. I've heard it appear on their videos. Sheesh.