MDI Aux-In USB Buzzing but iPod OK?

[ooMARKoo]

I have a 2010 GT which I believe had factory fitted MDI unit in the armrest and factory RNS510 radio. Sticker in the boot states 8AY which translates as Radio/navigation system and 9VE which is the dynaudio upgrade. It all seems like it was factory fitted.

When I bought the car (second hand from a VW dealer) the radio was not installed, this was common throughout the used cars they had in stock, they get them robbed I think. I guess it is faintly possible that a different unit was put back in the car, that's the first thing to note, but unlikely. Might this be related if the wiring loom is for a different variant of the head unit?

I cannot determine what model radio it is, I've read you are meant to hold down the SETUP button when powering on, doesn't work, think it needs a VCDS tweak to enable which I don't have - that's another story!

So, if I listen to music via the SDCard, CD, radio, onboard hard drive, all is fine and as expected. If I listen with an iPod using the MDI and VW adapter lead, all is fine and the tracks appear on the display.

If I try and listen to any device through the Aux-in 3.5mm socket, the volume is v.low (I've played with the Aux In Volume setting) and distorted/buzzy. You can listen and make out words in podcasts but it's dreadful. Similarly, if I try a USB stick in the VW adapter via the MDI same distortion. Using a microUSB lead in the MDI USB adapter to my Android phone, I can navigate the folders on the RNS screen and play them but same distortion. Finally with a 'genuine' lead from ebay that has the MDI connector direct to a microUSB through my phone, same result.

I find it very odd that the iPod works and the others don't. I have noticed that there are many more connections in the MDI plug for iPod than the others, my guess is it sends the audio streams via analogue on different pins? But with that in mind, why would the Aux-In (analogue) misbehave in the same way that the USB based devices do? Surely the USB is purely digital file transfer, why would interference be possible?

Car still has a few months of original warranty left and 10 months of dealer/VW used car warranty so I suspect it may be best to book it in but if anyone has real understanding of what is going on it would be helpful and good to pass on to the dealer so I don't get fobbed off.

Thanks.

[ooMARKoo]

In case anyone wonders, the dealer went through the loom and replaced the head unit (under warranty), it now behaves as it should. Integration with Android phones via a USB lead is a flaky affair though, sometimes it behaves and lists the tracks on the unit, other times it's not interested. I refuse to join the iSheep brigade so no silly suggestions please!