Very weird that you think your car is loud. The focus is known for how quiet it is.
I too have the engine cover but to be honest, I did not notice a change in sound - it was very quiet before installing, and still quiet afterwards.
Maybe something is wrong though? No way to tell without hearing it.
Mods: Drop in K&N filter, Steeda RSB, MS3 RMM, Remote start, Llumar tint, Ti engine cover, Husky floor mats, OEM splash guards