I heard the rattle AND had a strange trackpad problem, where the green selector box that appears when using the trackpad would randomly move around my screen untouched, often losing my place on a page, or randomly swiping my home page to the secondary screens. The trackpad wouldn't snap photos on occasion, too.
So, I took it to Verizon. They said "no phone should rattle" and thought it might be a speaker problem, shipped it off and replaced mine.
However, when my replacement arrived, I noticed this one, too, rattles. When the back is taken off, it's clear that the silver mic jack inset rattles. However, consistent with everyone's description, it rattles when tapped toward the top of the phone.
I haven't had too many call vibrations, but let me ask everyone this, what does it sound like on your phones when a set alarm clock alarm goes off during a phone call? Mine gives this God awful, high-pitched squeal.