Electro Magnetic Interference.
Basically, electricity coursing through wires can create fields of energy outside of those wires. Those energy fields can be picked up by poorly shielded (or even defective) equipment, and end up being unwanted sound.
Fixing it requires you first confirm it's there, and what it's from. Then, you either eliminate it, or get the amp far enough away that it's not affected.
While we wait for you to test: What amp, what car, what mounting location, what wires, where are they connected, are all questions that will help us help you.