engine noise can be alot of things not always the install, sometimes it is a bad RCA, could be a bad ground, a bad radio. we had a car that came in and had horrible engine noise and we did everything, but it didnt go away until we changed the pioneer radio and put a panasonic in, no more noise, not to say panasonic is better because i have had engine noise with a panasonic aswell. a quick fix would be to put ground loop isolators, but i do not reccomend that for the mustang because it has soo many amps and thats a lot of isolators...