Eliminate Engine Noise
So what types of noise are there?
In the case of car audio we are only concerned with induced noise or EMI (Electro Magnetic Interference). RF noise is not usually a concern in cars because it can be easily shielded against. EMI on the other hand is low frequency magnetic noise that cannot be shielded against. EMI can get into the system in two ways through ground loops and through induced noise into the audio cables.
Where does it come from?
Although our vehicles electrical system is powered by a battery, which should only put out DC our cars are usually used where our engine is providing the electrical power through an alternator. An alternator puts out an AC signal with quite a large amount of current. This is then rectified and placed on our cars electrical system to charge the battery and run the car. The alternator can cause a large magnetic field and is often the source for EMI in our cars electrical system. This usually appears as a whining sound that varies with engine speed. The high voltages used in our ignition system can also produce an electromagnetic field that causes noise. This usually appears as a ticking or clicking sound that varies with engine speed. If you have the latter type of noise you might want to check your ignition system and leads to make sure they are operating correctly.
How to remove it:
If you have alternator whine you are going to need a few components in order to find out where the noise is coming from. You will need some shorting plugs. These are simply RCA plugs/sockets with the two terminals shorted together. The first thing to determine is which speaker the noise is coming through. Once this has been done you can move to the amplifier through which this speaker is being driven. Remove all the RCA inputs from the amplifier and place the shorting plugs on all channels. Turn your system on. Has the noise disappeared? If it has then the amplifier and the associated wiring is OK and we can move to the next component. If the noise is still present then you may have some problems. Amplifiers usually have very good power filtering so you shouldn't have noise entering through these. Try another amp to see if it gets noise. Make sure the amplifier only has one ground (the proper power ground) and that the chassis of the amp isn't touching and metal of the car. If it is then remove it.
So we have decided that your amplifier isn't the problem. Let’s move down to the next component. Plug the RCAs back into the amplifier. Disconnect them at their source and place the shorting sockets on their ends (remember to do this for all channels). Is the noise there now? If it is then your RCA cables are the problem and they need to be replaced (maybe with twisted pair). They may also be damaged somewhere. Assuming your RCAs aren't the problem plug them back into their source. Now disconnect the RCAs going into this source and replace them with shorting plugs. If you don't have a processor or crossover then you can skip to the head unit stage. Turn the system on. Is the noise their? If it is then you have found the problem. Once again make sure this unit has only one ground and remove all others. This device may have poor filtering or a poor design. In which case you may need to replace it with something else. Some units are notorious for creating noise problems. There are several tricks you can try though to help alleviated the noise from this unit. A power filter on the supply might help. Also a ground loop may be forming through the output RCAs. You will have to remove one of the grounds on one of your RCA cables to defeat this loop. This can be difficult and may be something you don't wish to do or you can add a RCA ground loop isolator.
If you haven't found the noise yet then continue to the next component. If you find that your head unit is the problem then try removing from the front of the car and take it down to where the amps are and play it from here. Chances are the noise is not there. In this case move it back to the front of the car. Once again make sure that it only has one ground. This can be hard if it mounts into the metal of the chassis. See if you can run it near where it is usually mounted but not touching the metal of the car. If he noise is not there then this ground loop is probably the problem. If you still get noise then you might want to add a power filter to the head unit's power. The head unit may be a poor design. To check for this you can replace it temporarily with another model or you can take the RCAs out from the head unit and place some RCA shorting sockets onto the end. If the noise goes away then you know that your head unit has bad filtering or has high output impedance. Either way it may be time to replace it.
Remember that twisted pair RCAs will only fix your problem if you have induced noise through your cables. It will not fix the noise if you have a round loop or faulty components. You can read the twisted pair tutorial if you want to know why twisted pair cables can be superior to the normal RCA cables when it comes to noise rejection.