Identification Of Noise Source

WARNING: This page is about a different car, the 2006 Toyota Tundra and 2005 Toyota Tundra. However, it is still accessible from the selected car via links, so may be relevant.

RADIO WAVE BAND
The radio wave bands used in radio broadcasting are as follows:

Fig 1: Radio Frequency Band Chart
Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
SERVICE AREA
There is a great difference in the size of the service areas for AM and FM broadcasting. Sometimes an FM stereo broadcast cannot be received even though AM can be received very clearly.

FM stereo has the smaller service area, it also picks up static and other types of interference ("noise") easily.

Fig 2: Identifying Service Areas Of AM And FM Broadcasts
Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
RECEPTION PROBLEMS
HINT:

Besides the problem of static, there are other problems, such as "phasing", "multipath" and "fade out". These problems are caused not by electrical noise but by the nature of the radio waves themselves.
1. Phasing
  Besides electrical interference, AM broadcasts are also susceptible to other types of interference, especially at night. This is because AM radio waves bounce off the ionosphere at night. These radio waves then interfere with the signals that reach the vehicle's antenna directly from the same transmitter. This type of interference is called "phasing".
  
  Fig 3: Identifying Phasing
  Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
2. Multipath
  Interference caused by reflection of radio waves against obstructions is called "Multipath". Multipath occurs when radio signals emitted from the broadcast transmitter antenna are reflected against tall buildings or mountains and interferes with other signals which are to be received directly.
  
  Fig 4: Identifying Multipath
  Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
3. Fade Out
  FM radio wave tends to be reflected against obstructions such as tall buildings or mountains because FM frequency is higher than AM. For this reason, FM signals often seem to gradually disappear or fade away as the vehicle goes behind those obstructions. This phenomenon is called "fade out".
  
  Fig 5: Identifying Fade-out
  Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
NOISE PROBLEMS
It is very important for technicians to clearly understand a customer's claim about noise. Use the following table to diagnose the phenomena.

Fig 6: Noise Occurrence Condition - Presumable Cause Chart
Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002

HINT:

If the condition when the noise occurs does not meet any of the above, find out the cause based on "Reception Problems". Refer to the description about Multipath and Phasing mentioned previously.

Identify the condition in which the noise occurs, and check the noise filter on the related part.

NOISE OCCURRING CONDITIONS AND NOISE SOURCE

Condition in which noise occurs	Noise Source
Depressing the acceleration pedal increases the noise, and stopping the engine stops the noise immediately.	Generator
Noise occurs during A/C or the heater operation.	Blower motor
Rapid acceleration while driving on an unpaved road or after the ignition switch is turned on makes noise.	Fuel pump
Pressing and then releasing the horn switch, and keeping pressing the horn switch makes unusual noise.	Horn
Quiet noise is heard while the engine is running, but stops with the engine.	Ignition
Noise occurs synchronously with the turn signal flash.	Flasher
Noise occurs during window washer operation.	Washer
Noise occurs while the engine is running, and it continues even after the engine stops.	Engine coolant temperature sensor
Noise occurs during wiper operation.	Wiper
Noise occurs when the brake pedal is depressed.	Stop light switch
Others.	Static electricity stored on the vehicle

Reference:

Make sure first that there is no noise from outside. Failing to do so makes the noise source detection difficult and leads to misdiagnosis.
The noise should be removed in descending order of loudness.
Tuning the radio so that no station is received wakes the noise more noticeable, making the recognition of the phenomenon easier.