My first thought was excessive AC ripple from the alternator. If you have a digital voltmeter, it's quick and easy to rule this out. The first link is for Mercedes, but the prinicples are the same on all modern brands:

http://www.pvv.org/~syljua/merc/TooSeptST07.pdf

http://assets.fluke.com/appnotes/aut...e/beatbook.pdf


Excessive AC ripple can cause all sorts of strange symptoms, but with no corresponding warning light or fault message. I would suggest spending 5 minutes to quickly rule out this possibility.

If at all possible, take the reading directly at the alternator, not the battery. Red lead on the big output wire. Black lead on the alternator case. Set the meter to AC (not DC) volts and start the engine. Select the lowest range that will let you see 0.5VAC. Anything more than that is a bad alternator.

Actually, you can start with a reading at the battery. If it fails, then you can skip the reading at the alternator, as you know it’s bad. But if it passes at the battery, repeat the test at the alternator to be sure.

One more thought: The noise may be coming through the reverse parking system, not the radio. Try switching off the reverse distance sensor (switch on overhead?) and see if that helps.