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Interference noises due to power supply

WARNING: This page does not describe the selected car, but rather 7 other vehicles, including the 2005 Porsche 911, 2004 Porsche 911, 2003 Porsche 911, 2002 Porsche 911, and 2001 Porsche 911. However, it is still accessible from the selected car via links, so may be relevant.
NOTE: Use this test routine for interference noises which originate from the vehicle electric.
RADIO SYSTEM DIAGNOSIS FOR INTERFERENCE NOISES DUE TO POWER SUPPLY

Work instruction Yes No
1
  1. Temporarily disconnect the connections B+ and ground on the radio.
  2. Connect B+ and ground temporarily via a separate lead.
  3. Connect the other end of this lead to the vehicle battery.
Interference noise still present?		 The interference noise is triggered via the radio. Step 2
  1. Reconnect the original cable connections to the radio.
  2. Temporarily disconnect the connections B+ and ground on the booster.
  3. Connect B+ and ground temporarily via a separate lead.
  4. Connect the other end of this lead to the vehicle battery.
If the noise is still present, the amplifier is the cause of the fault Step 2
Noise no longer present Step 3
2 Connect the other end of the improvised lead connection at the connection B+ to the fuse of the radio or the amplifier.
Interference noise still present?		 Lay B+ and the ground lead differently or insert a new lead and lay the new lead at a distance from all other current circuits.
Noise still present: Step 3
Noise no longer present: End		 Step 3
3 Check whether the noise occurs when a switch or an accessory component is switched on. Localise the cause for the interference noise in the retrofitted accessories and remedy. End The noise occurs when the engine is running. Step 4
The noise occurs with the ignition switched on and the engine at standstill Step 5
4 Briefly disconnect the connections on the alternator and start the engine. Noise still present?    
5
  1. Find out which circuit is causing the interference noises by removing individual fuses, relays and switches.
  2. Insert all removed fuses, relays and switches again.
  3. Disconnect all components in turn which are connected to the affected circuit, until the interference noise has disappeared.
  4. Replace defective components.
Noise still present?		 Step 6 End
6
  1. Lay all wiring harnesses from the defective component and the connecting leads with suitable clearance.
  2. Remove any audio cables which are causing the problem from the wiring harness.
Noise still present?		 Step 7 End
7
  1. If the suspected components uses the same ground connection as a component of the audio system, establish a problem-free ground connection.
  2. If ground connections are running via the same wiring harness, separate them from each other.
  3. Wrap the B+ lead of the suspected component with a film or a metal tape and connect the lead to a ground connection.
  4. Check the ground connection with an ohmmeter at the negative terminal of the battery.
Noise still present?		 Step 8 End
8
  1. Find the positive cable of the suspected component and cut the cable at the component side.
  2. Insert an interference filter between the connecting cables.
 End End
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