2009 Jetta TDI humming with iPod plugged into 12v

Hello, My dealership is giving me the run around. I have a fully warrented 2009 VW Jetta TDI with a top of the line factory Nav. When I plug my iPod into the AUX port it plays great. When I then plug the iPod into AUX and 12v lighter. I get feedback that goes up and down with RPM. I have tried 2 iPods and and iPhone and 3 different sets of cables (both 12v and AUX). The same does not happen with my wife’s Ford Edge. The dealer says since it is intermittent it must be my cables, device, or MP3 files. What can I ask them to check to confirm what is wrong.

A ground loop isolator adapter ($13) all but eliminated the problem so I really don’t care but I would love to rub thier noses in it. I’m thinking its a bad ground (alternater, radio, or 12v) but really don’t know were to look or what to look for. Any suggestions are welcome.

I think it it may be more of a problem with a cheap power adapter that doesn’t provide adequate filtering. Or the vehicle’s alternator may be introducing some AC ripple into the car’s systems.

Does the IPOD do this on another vehicle with an AUX input (not a VW) when operated in the same way?

Thank you for the reply. I do not recieve the feedback on my wife’s car (or a couple others I tried). I also consistently get the same feedback even though I have tried 3 different adapters and AUX cables (all different manufacurers). Is there a way to test for a badly grounded alternator.

Dealers aren’t that sharp with audio electronics, they simply replace defective radios, CD players, and Nav systems. A good car audio shop can easily identify the source of your interference. It may the be alternator, but it could be in the rectifier or other circuits that are part of the charging system.

Some cars are just “noisey” and there may not be a “defect” in the operation of the alternator and/or charging system. I’d rather go with a simple filter solution than have a dealer start replacing parts and perhaps get nowhere and do more harm than good.