Or try a ground loop isolator.
Commonly called alternator whine, your noise was actually caused by an internal connection between your laptop's power ground and audio signal ground. Current flow over the audio ground wire through your head unit caused a voltage difference between the laptop end of the audio cable and the head unit end of the audio cable. With the engine off, this static difference is pretty harmless, but once the alternator begins sending a very dirty waveform of voltage to the electrical system at high frequency, that voltage difference between the two ends of the wire oscillates with the alternator and since the left and right channel wires don't oscillate along with the ground, the voltage difference ends up making it to the speakers.
The end result of all that is: the frequency being output by your alternator winds up coming out your speakers. When you added a ground loop isolator, the current travel over the ground wire stopped, and so did your noise. The ground loop isolator is nothing but a 1:1 transformer, which means that a/c (sound waves) get through and d/c (power consumption) doesn't.