It could also be your amp's power wire picking up static bleeding off your spark plug wires. You can check your engine-to-frame, engine-to-body grounds for corrosion or deterioration, or just missing connections haha, those grounds being faulty can cause static buildup in the engine. It can also be the static bleed-off on your coil....clamp a jumper cable to the bracket surrounding your coil's primary and clamp the other end to your negative post on battery, see if that lowers the sound any.
Back in the day, noise filters were more common due to the older points ignition vehicles because there usually wasn't alot of protection against static/electricity bleed-off, that would find it's way into the headunit one way or another, either by the power wire or by the antenna cable.