I noticed the same rattle on my brand new MBP 15" Retina and found this post. I looked at the internal pics of this Mac and it looks like the only thing that could be rattling under the palmrest is the battery. It is a distinct sound of something being loose. I noticed it first when I picked up the Macbook and it sounded like something was broken. The iFixit teardown explicitly shows that glue is used to hold the batteries down. My assumption now is that the glue is failing and the batteries have come loose. I can easily reproduce when holding the Macbook open on my lap and tapping on the front right and left corners of the palm rests. If I lightly shake the open Macbook left to right I can also hear something inside shifting left to right. My hope is that this is not intentional and is some sort of manufacturing mistake. I will be showing this issue to a genius bar this week.