i'm pretty sure it's normal. As far as i can tell, it seems to just be the exhaust system moving with the engine and bouncing around in the rubber bushings that hold it in place. Either that, or it's also just that it's a 3-cylinder running at low rpm's.
And I, too, have a Brabus, and absolutely love it. Especially since i got a cold air intake and completely redid the sound system. haha