The reason that cars are noisy is that magnetic fields are created in any region of space where a magnetic force is exerted. For instance, in the region of the alternator, large magnetic forces create the potential needed to charge our starting batteries and operate our 12-volt accessories - including the car stereo system. The last three hundred years of physics have taught us that the strength of the magnetic field is at a maximum when the force on a moving charge is perpendicular (at right angles) to the magnetic field. There is no force when the moving charge is parallel to the field. This means that the magnitude of the force varies between a minimum and maximum, as the moving charge is orientated with respect to the field. In car audio, the moving charge can be the electrons moving in a signal cable or in a component. Magnetic fields are everywhere, but the car's chassis can be considered as a large plate with its magnetic field stronger nearest the chassis and weaker as the distance between the chassis and the moving charge increases. So the magnitude of the forces exerted upon the moving charge is dependent on the charge's orientation and distance with respect to the field. In practice, we know that if the route of the signal cables is changed, or if the distance between the signal cables and the car's chassis in changed, the level of noise induced into the audio system will be changed. By the way, the "(magnetic) force on the moving charge" translates into alternator whine for car audio systems.