Just got my rMB Pro 13 TB and it *has* the "rattle", but I wouldn't call it a rattle. It is an almost imperceptibly gentle thud and, especially now I've read on here advice that the plastic sheet needs to be able to expand when it warms up, I honestly couldn't care less. It may be a design feature, it may not, but it is so slight - and only occurs when I purposely tip the thing and listen carefully - that it makes zero difference to me. As someone said a few posts above, a Macbook without the thud may develop it anyway over time after an unknown number ofexpansion/contraction cycles... The looseness of the plastic sheet has a purpose of sorts. Let it be; find something bigger to worry about ;-)
For anyone who is thinking of letting this issue put them off purchasing, don't pay any attention to this issue. The sound is like an old-school overhead projector A4 plastic slide tapping gently when shuffled in a pile like a pack of cards. Really nothing.
The almost imperceptibly gentle thud: