First turn off the engine and check whether if there is any noise.
If no noise
Start the engine and check.
If noise appears,
most probably the reason would be improper grounding.
Most car audios have outer casing and inner casing. Some people connect the ground cable to outer casing for easy. (They don't use the power connector ground wire.) As a result the circuit board ground doesn't receive 0 volts exactly and noise arise. So correctly ground the unit using shorter length wires will minimize the noise.
If this not works then another reason would be the problem with the input filters of car audio power supply. For this you have to add LC filter before the car audio power supply.