My friend has an 04 Elantra gt, and had one of his friends install the system (i was out of town). The system sounds great, but has the imfamous alternator whine. It is rpm dependent, but heres the weird part... the whine only comes on when his xm is turned on. It is a alpine headunit, and the xm is connected through ai net. Even when i pull the rca's out of the xm box, the whine is still there, but if i source the headunit to fm or cd, the whine gos away. I have tried different grounds on the amp, turning the gain down, etc.... i just dont get why it only does this thorugh xm... any help is appreciated.
The signal cable running from the xm reciever to the alpine cd player could be picking up noise from a nearby power cable. Try isolating the signal cable away from all other cables and see if the noise goes away.
I had the same problem with my wife's Sirius radio that is connected to the stock deck with rca's. I ended up using a radio shack ground loop isolator and it went away.
https://www.radioshack.com/product/index.jsp?productid=2062214&cp=&kw=ground+loop&parentPage=searchI woudl try isolating the cable before you buy the ground loop isolator. Those are more of a band aid then a real fix.
In my wife's mini cooper it is caused by running the sirius radio from the cigarette lighter for power. It does it in all the mini's, and the common fix is to use the ground loop isolator.
Try using a different power source if you can and see if the whine goes away.
Make sure the xm piece is seeing the same voltage (power and ground) as the other equipment.
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My ex once told me I have a perfect face for radio.