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 only affect the power wire for the amplifier (and anything else that is tied in with it), and is induced noise. Some sources of this are: the distributor, the ignition coil, the plug wires, and the spark 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 and spark plugs are RFI (Radio Frequency Interference) shielded.

Induced Noise

Induced noise is created when it is radiated into a cable or component. To check for induced noise, leave the head unit hooked up, but move it as far out of the dash as the cables will allow. If the noise stops when the unit is out of the dash, but returns when it is placed back into the dash, then an electrical component is radiating noise directly into the chassis of the head unit. If the noise does not diminish when the head 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 similar devices. 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 like a microphone (You can also use a portable AM radio as a sniffer, 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 suspect any factory cables in the area of the receiver. In the 1985 Ford F-150, for example, there is a ground wire that is routed directly above the radio. Once you find the offending wire(s), you will need to re-direct them away from the receiver. If this does not completely clear up the noise, then you can shield the receiver with a layer of high-iron content metal (such as MuMetal). In the case of the RCA cables, it is usually easier to re-route the cables rather than to relocate the noisy component.

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 their output 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 componnet in the car, or even from 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 for this is to unhook one speaker at a time from the amplifier. If the noise stops when one particular speaker is disconnected, then that speaker should be replaced with either a different brand or a different size speaker. This will provide a different sized voice coil, changing the frequency it is able to pick up. In almost all cases, this should eliminate this problem.