MBPr strange internal sound

Hi everyone and sorry for my bad english

i've got a problem with my macbook pro retina late 2013, when i shake it a little i can hear something inside that moves; don't know how to explain, it sounds like something big.

I just checked in the back and all screws are there, so i don't know what this should be...
As i know there isn't any mechanical parts in there (only fans) so i don't have any ideas. Anyone noticed anything like me?
i never tried to open it, and was thinking to get it to support.
Thank you