It is probably related to some part or the suspension being worn. The front strut mount bearing are know to wear and produce a lot of noise. Check all your suspension components and make sure they are all properly lubricated.
there is a technical service bulletin on 04-06 focus's with 15" 9 spoke wheel covers making noise. the fix is to put a piece of hard foam tape by each of the hubcaps retention legs near the edge of the cover.
It probably needs to be lubricated. If it creaks on braking you may have worn brake pads. If it creaks while turning the steering wheel you may need to lube your tie rod ends or the bushings in the steering column.