Crutchfield

If you have a whining noise in your system that varies in pitch with the RPM of the engine (and disappears when the engine is off), then you are experiencing alternator whine.

The alternator on your car produces Alternating Current (AC), but then converts it to Direct Current (DC) by means of a bank of diodes known as a rectifier bridge. The noise that you hear from the alternator is known as "ripple", and is caused by some of the alternating current slipping past the diodes. It can be picked up by components either over the power supply, over the signal path, directly through the case of a component, and through control or output leads.

Some of the major causes of ripple being produced by the alternator are:

Ground Loop - A difference in ground voltage between the Head Unit and the Amplifier.

• Extend the Head Unit's ground wire to the Amp ground location. Ground everything in the audio system (equilizers, crossovers, etc) to a single point near the Amplifier to eliminate the ground loop.
  

Defective Diodes in the Alternator - The diodes in the alternator could be defective. This can be caused by:

• Heat
• Overcharging
• Jump-Starting
• Foreign Material

• weak.
• overcharged.
• undercharged.
• dirty.
• not making a good ground connection to the vehicle chassis.

• Dirty headlight bulb connectors
• Clogged wiper assemblies
• Poor electric window systems
• Defective Alarm Systems
• Accessory Lighting (such as neon and fog / driving lights)

One main point to note on the alternator - It's production of alternating current is three-phase, meaning that it is produced in three separate cycles. If one of the cycles were to be completely blown out, the alternator would still be able to produce current, it just has to work harder to keep production at the same level. Having to work harder also means that it increases it's noise production as well.
If you have reason to believe that any of the above may be causing you to have a faulty charging system, then a quick method of checking would be to pull a known noise-free car alongside the suspect vehicle. Then, with the known-good vehicle's engine running, connect a jumper cable between the positive posts of the two batteries and the other cable between the alternator brackets of the two cars. Do not start the suspect vehicle, just operate its stereo system. Does it still have whine? If not, the car may very well have a faulty charging system.
If it does still have whine, then it is time to figure out where the noise is getting into the system. The majority of alternator whine problems can be traced to bad signal grounds. If unplugging the RCA cables makes the noise go away, then the problem is that the noise is entering over the signal path. A single grounding point for both the left and right signal grounds is the most important aspect of a noise-free signal path. The reason that we want only one grounding point is that if you only have one ground point, a ground loop cannot exist.

Ground Loops

When any accessory uses the body of the car for ground, other accessories that use the car body for ground can form a common return path. In these cases, some accessories can limit the amount of ground that other accessories can receive. This is commonly known as a ground loop. If the amount of ground return is affected, then the amount of power into the component is affected. In autosound components, this can greatly affect the amount of power output, as well as cause noise.
The best place to establish a common Signal Ground is at the back of the head unit. Since the head unit has to be at one end of the system, and the amp is at the other end, it is a great place to establish the one-and-only signal ground. This is commonly taken care of automatically through almost all modern head units, by internally making the same reference to ground for both left and right RCA shields. The main goal is to make sure that this ground gets to float all the way to the amp(s), and not get misdirected along the way. The same signal ground seen at the deck must carry down through the rest of the components to the amplifier. If the grounds are different between any two components, ground noise can creep into the system. Most aftermarket autosound component manufacturers help you keep a single signal ground by floating the component's signal ground and allowing all grounds to reference back to the head unit. Sometimes, though, the signal output grounds will be chassis grounded on an individual component. If that component does not use DC-to-DC converters in it's power supply to isolate that ground, their signal outputs will be grounded and noisy. It is usually easier to substitute a more expensive component (with no chassis ground on it's output) than spend an hour trying to trace the whine. If there are several suspect components in line between the head unit and the amp, just take a long RCA cable and run it straight down the middle of the car (over the carpet, seats, etc.) straight to the amp. If the noise goes away, then it's time to check each individual component. One method of finding out which component is responsible is to bypass one component at a time using double female RCA barrel connectors. The barrel connector will float the outer shield of the RCA, preventing a second reference ground and a ground loop. If the noise goes away when you bypass a particular component, then you have found the culprit.
As for Power Supply Grounds, it would be a good idea to have all of the components grounded to the same spot as well. For this, the best spot is at the amp location. The amp will normally need the most power of any component in the system,, therefore it's ground cable needs to be the shortest (in order to have the least resistance). For the head unit itself, you would want to make sure that it is not grounded anywhere up in the front of the vehicle. Instead, you would want to extend the unit's ground wire (using the same size wire as what comes on the unit's harness)all of the way to the amp location, and ground it to the exact same point that the amp is grounded. The reason for this (as with Signal Ground loops), is that if there is any difference in ground potential between any of the components in the system, it can cause a ground loop and draw noise in over the power supply. All powered accessories need a ground to work properly. If these components are trying to use the same ground path through the body of the car, then this can cause ground loss in any other component that is hooked up to the same ground path. When this happens, the unit does not work as efficiently, and in the case of autosound systems, can cause noise as well. (This is one explanation for why a car's headlights may dim on deep bass hits; because the headlights and the amplifier that is running the subs are on the same ground path. It can also be because the sub amp is pulling so hard on the alternator that it can't supply enough current to run other accessories properly.)

Induced Noise

Another cause of noise in the system is induced noise. Induced noise is basically just noise that is radiated into a cable or component. In the above instance of unplugging the RCAs and having the noise disappear, another reason could be that noise is being induced into either one of the components through it's location, or due to being induced into the RCAs directly. To check for induced noise, first just try leaving the head unit hooked up, but move it as far out of the dash as the cables will allow. If the noise suddenly disappears when the unit is out of the dash, but returns when it is placed back into the dash, then something is radiating noise directly into the body of the head unit. If the noise does not disappear when the unit is out of the dash, then try running a different RCA cable straight down the middle of the car into the amp. If the noise then disappears, the noise may be radiating directly into the RCA cable itself. If so, try re-routing the RCA cable in the car so that it runs as far away as possible from the power wire for the amp, any factory computers (such as the Body Control Module or Engine Control Module), heater and A/C fan, factory relays, or anything similar. If you are sure that it is radiated noise, but can't seem to move the cable or component away from it, then you may need to use a noise sniffer. A noise sniffer is simply a cassette Walkman with an extension wired in so that the tape head can be held out basically like a microphone (You could also get similar results with an AM radio, though the results will not be as precise). You would want to turn the volume all the way up, put the headphones on, and "sniff" away at the entire route of the RCA cable. In the case of the head unit, you will especially want to look for any slightly larger gauge ground wires. In a 1985 Ford F-150, for example, there is a ground wire that is routed directly above the radio. If you find the wire(s), you would need to cut it, and solder in a large gauge (8 gauge would be recommended) extension, being sure to re-route it away from the radio. If this does not completely clear it up, then you can shield the radio with some sort of high iron content metal (such as MuMetal). In the case of the RCAs, it's always easier to re-route the cables than to relocate the noisy component.

Ignition Noise

If the noise has a ticking associated with it (that varies with the RPM of the engine), then you may be experiencing Ignition Noise. Usually, this would just affect the power wire for the amplifier (and anything else that is tied in with it), and this is typically induced noise as well. Some sources of this are the distributor, ignition coil, plug wires, and plugs. If you have ignition noise, you would want to move your power wire as far as possible from any of these sources. If that doesn't help, then you would want to check all of these components to make sure they are not faulty, and make sure your plug wires are RFI (Radio Frequency Interference) shielded.

Speaker Coils

If you have a noise that cannot be solved by any of the above methods, then you may have a Negative Feedback Loop. Many amplifiers are designed so that they sample part of their output signal at the speaker to make adjustments to their amplification. It is possible for the speaker coil to work like an antenna for noise, whether it is radiated by another component in the car, or even radio reception. If this is the case, then it is possible for this noise to be passed back into the amp, and out to the rest of the system. The easiest way to check if this is the case is to disconnect one speaker at a time from the amplifier. If the noise suddenly goes away when one particular speaker is unhooked, then that speaker should be replaced with either a different brand or a different size speaker. This will provide a different sized voice coil, thus changing the frequency it is capable of picking up. In almost all cases, this should eliminate this problem.