the secondary inlet runners, they open around 3900rpm at light throttle and around 3400rpm at wide open throttle you should feel them operate when booting it. they can get gummed up and also the inlet maniold runner control (imrc) can fail
some info on fixing a failed one here http://www.fordmondeo.org/wiki/index.php/Fix_IMRC
you may have to register, its free but there is a wealth of info on there and worth it. if you do i'm on there as d c