When looking around to do the system on my Mondeo I had an "open chamber" resonator recommended to me to stop drone. Marathon mufflers in Qld custom make them and they are supposed to be excellent for problems such as yours.
I haven't had my resonator replaced yet, so can't comment on the effectiveness.
'00 Mondeo ST24 2.5l Duratec V6
custom CAI incl K&N pod filter, flowed throttlebody, exhaust, Koni Sports shocks, EBC Greenstuff pads, Whiteline rear swaybar.... Mondeo ST24 Information Site