I have the same problem and I found [http://forum.xda-developers.com/nexus-5x/help/rattling-noise-near-speaker-t3231268|this thread @ xda-devs]
Quoting the author:
'''SOLVED! - I spoke to Google support and they said that the rattling noise comes from the sim tray.
There is a tiny lever which you push with the sim pin to remove the sim tray, and that lever is what rattles against the frame. Providing that your 5X works as it should, that's the rattle.'''
I wouldn't worry about it.
Best of luck!