I'm sorry, could you please specify your budget? And what kind of build visual are you looking for, like a strong, metallic, solid sturdy design, or more of a cheaper plastic overall build?
now that i put even more thought into it, I'm starting to think that it might me your central processor? I don't really know though...there might even be a chance that is is both the hard drive and the processor.
It is your Hard Disks, you probably have it lose or it doesn't have enough room so it scrapes against something internally. The only recommendation i have is to try booting of of another External hard disk and see if you still here the shuttering. If you do i don't really know what to do but if...