Identification Of Noise Source

WARNING: This page is about a different car, the 2008 Toyota Avalon. However, it is still accessible from the selected car via links, so may be relevant.

Radio Description

Radio frequency band
1. Radio broadcasts use the radio frequency bands shown in the table below.
  
  Fig 1: Radio Frequency Bands Reference Chart
  Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
Service area
1. The service areas of AM and FM broadcasts are vastly different. Sometimes an AM broadcast can be received very clearly but an FM stereo cannot. FM stereo has the smallest service area, and is prone to pick up static and other types of interference such as noise.
Radio reception problems
HINT:

In addition to static, other problems such as "phasing", "multipath", and "fade out" exist. These problems are not caused by electrical noise, but by the radio signal propagation method itself.

Fig 2: Identifying Service Areas Of AM And FM Broadcasts
Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
1. Phasing
  AM broadcasts are susceptible to electrical interference and another kind of interference called phasing. Occurring only at night, phasing is the interference created when a vehicle receives 2 radio wave signals from the same transmitter. One signal is reflected off the ionosphere and the other signal is received directly from the transmitter.
  
  Fig 3: Identifying Radio Phasing
  Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
2. Multipath
  Multipath is a type of interference created when a vehicle receives 2 radio wave signals from the same transmitter. One signal is reflected off buildings or mountains and the other signal is received directly from the transmitter.
  
  Fig 4: Identifying Radio Multipath
  Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
3. Fade out
  Fade out is caused by objects (buildings, mountains, and other large obstacles) that deflect away part of a signal, resulting in a weaker signal when the object is between the transmitter and vehicle. High frequency radio waves, such as FM broadcasts, are easily deflected by obstructions. Low frequency radio waves, such as AM broadcasts, are less likely to deflect.
  
  Fig 5: Identifying Radio Fade Out
  Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002

Noise problem

Technicians must have a clear understanding about each customer's noise complaint. Use the following table to diagnose noise problems.

PROBLEM SYMPTOMS CHART

Radio Frequency	Noise Occurrence Condition	Presumable Cause
AM	Noise occurs in a specified area	Foreign noise
AM	Noise occurs when listening to an intermittent broadcast	An identical program transmitted from multiple towers can cause noise where the signals overlap
AM	Noise occurs only at night	Signal phasing
FM	Noise occurs while driving in a specified area	Multipath resulting from a change in FM frequency