hi, when you get hums or whine noises that you hear from car audio systems, it is caused by electrical interference. most good head unit should not emits any of this audible noise. by saying that, your car's electrical system can also be the problem, old components may create a fault or picked up some sort of interference and this is put straight back through the car's power lines powering your head unit.
Old and tired batteries can cause this problem also.
a good car audio installer will use your factory wiring looms, why? because these cables are dedicated for your sound system and hence your head unit is not crossed wired to anything eles. also solder all contacts and heatshrink/taped all bare wires as it can easily pick up interference from current running through other wires.
noise suppressor can sometimes help and you can get them from Jaycar