Trackle noise from the Rear armrest

Hello, I have some trackling noises coming from the rear armrest. It is some piece of metal or plastic fallen into and it is very annoying when going trough lumps or bumps on the road. 

I can`t open it it is inside the plastic part which separate the trunk from the rear space. 

Can you tell me how to open the armrest it self not only the falling part.

   Thank you !