Help: rolling ball sound on passenger side door

[Dai]

I have a sound which can be described as if a ball rolling inside my passenger side door. This only occur when I stopped quickly and when I accelerate quickly. There is nothing inside my side pocket. I wonder if there is something loose inside the door. My window works just fine and my car is 2001 Boxster S. I also would like to add is that the sound could be under the door but difficult to identify because I usually drive alone.

Any insight on this would be greatly appreciated. Thanks for your help in advance.

Regards,
Dai