It would seem like somewhere on the internet there is a fix for this, as so many new production cars have auxiliary inputs. Wouldn't seem like this is just an EVO X problem.

 

I've identified two sources of sound in our aux inputs, a ground loop issue as well as an EMI issue caused by electrical noise and proximity of the aux cabling to power cables on the car.

The ground loop noise is a constant hum that is unrelated to vehicle RPM. As stated before, this is because your phone is grounded via two different paths, through the aux cable and the power cable, which causes the hum. I am still working on this.

The other noise, the alternator whine which sounds like a whistle at high RPM's, can be solved with Ferrite beads on the aux cabling. I put one on the cable leading into the aux adapter, as well as the cable behind the connector in the dash. This seems to all but eliminate the alternator whine.

This is what a Ferrite bead looks like, you can get them on Amazon, Fry's or Radio Shack:


Good luck! As for the ground loop issues I'm having, a ground loop isolator helps with but doesn't completely eliminate the problem. I've had better luck with a headphone amplifier wired inline with the aux, which seems to help by increasing the signal to noise ratio.

 

On a slightly different note if you're using your phone as an MP3 player make sure the quality of th MP3's loaded is no less the 160kbps.

I prefer 192kpbs or higher for the least amount of distortion and crackle from playback which is really evident at higher volumes and with a system.

As mentioned you're probably having a cable issue but it doesn't hurt to listen to quality stuff.

 

is your iphone/mp3 player plugged into the lighter / power? I had some weird noise when plugged into power on my sony walkman mp3 player. If playing it off of battery, no issues, if playing it while plugged in, gave horrible noise.