Sheppers / rattles - it annoys !

Hello.
with my recently acquired V8 I have for a few days the following (nervous) problem:
When a gear is inserted, you hear an even shuffling / flapping all the time - as if the exhaust was loose.
If I put in N or P, the sound is gone...
I didn't notice it until I opened the window during a city drive. But it's really annoying when the walls still reflect the sound.
What can that be? You can't simulate it on the lift - take the driving stage and let the engine run is probably a little bad.

The sound comes in my opinion from the front (in front of the passenger cell).