I hate rattles myself too. I will going to my local Home Depot and looking for thin piece of felt tape, to make sure that any areas I find expose to a potential rattle hazzard, it will be plugged. Same with the wire. Use of zip ties, and felt tape around the wires will help take care of potential issues down the road.
I will trace that wire as far as I can through the little hole, and take pix to share with you, if possible.

I ordered the RS6 from factory with the Navigation.
I am not sure if the car did not have this option, if it is any harder to get the RNS-E to work. I assume the GALA connector that Mike is talking about should be available on all RS6 vehicles.
In my case, I'm going with the antenna already in place. You can by a new antenna connector, and house it in the cuby hole behind the instrument cluster as most have done. However, the folks at NavPlus.us would best know if this addition can be done to an RS6 that did not have the regular navigation from factory.

With the RNS-E, I will still get road directions from voice, and little rectangle within instrument cluster. But now I will also have colored screen directions which is better visually, MP3 listening capabilities via dual memory cards and CD as well (but not on DVD), and DVD maps (1 DVD instead of 9 CDs).
The CD changer is gone, as now CD and DVDs are read from same slot. XM is still there, via new module. I can add a CD Changer in the back (possible future option), but MP3 reading from CDs can only happen from the in-dash slot, and not from CD Changer. You can opt to go with Phatbox instead of CD Changer too. Why not!!!