Why does my phone make a weird noise when I shake it?

[AC Question]

If I put my fingers on the screen and the back glass and shake my phone back and forward it makes a weird noise it doesn't affect the phone in any way I know a few other people who have the same phone and theirs do it as well I was just wondering what it is.

 

[Tim1954]

The vibrate function has a little offset cam in it, maybe that's it?