i will say my focus is very quiet inside w/ no rattles at all.. well, in the beginning the headliner made noise but a few well placed pieces of foam stopped that.
the only noise i really hear is at hwy speeds and that's coming from the rear view mirrors. they are big and i find they create alot of wind noise. neither my civic nor my older focus has any noise from the outside mirrors... I think the mk3 focus mirror is just a bad design vis a vis wind resistance tho it is nice and big to see out of.