2007 Audi A6 Vibration Over 60mph
 

I purchased a 2007 Audi A6 (C6) in the beginning of November. After test driving a few on my search for a new (to me) car I was hooked. The car was powerful, quiet and rode so nice. Now that I purchased one I don't know what I was thinking.... So far the passenger side adaptive head light failed, rear passenger windows with no longer works and now I'm getting a vibration when driving. It is very faint under 60mph, but increases in intensity as I approach 65mph where it is the strongest. It is more noticeable at part throttle, cruising. The intensity starts to fade a little as I increase in speed over 70mph.

I thought that it was a tire balance issue. I took the car back and had the tires rebalanced twice, but it was still there. Then I rotated the tires to see if the vibration felt like it moved, but it feels the same. Tonight I pulled the car in the garage and put it up on jack stands. I put the car in drive and carefully increased the speed until the vibration was present. Up on jack stands the vibration started to noticeably come in around 30mph. Next I removed all four tires and tried again. The vibration was still present and the intensity was almost identical. At this point I assuming the tires are eleminated as the issue. I grabbed a creeper and rolled under the car to start inspecting the drivetrain. Everything looks good and tight with the exception of the front passenger side axle shaft. The mounting flange for the driveshaft seems to have more lateral (up/down) movement than the driver side or rear axle shaft does. The seal is in good shape and is not leaking.

Has anyone had this or a similar issue? What was the issue and what did you do to fix it?

I had a similar issue on mu c5 A6 and after replacing almost everything I could think of - it turned out to be the sway bar. However just because it ended up being that for my car, doesn't mean that is your issue. You have to inspect and start replacing anything that could be at fault, here my list of things I tackled trying to resolve this:
1. tires (you checked that thoroughly) - it did improve somewhat after getting 4 new tires and a balance.
2. axles (check CV boots)
3. control arms
4. wheel bearings
5. struts
6. alignment
7. sway-bar
This was the order in which I worked my way through and of course it was the last item.

For your other 2 issues I would check this:
* Adaptive headlight:
a. bulb (on my s4 a bad bulb can cause the adaptive headlight to fail), not sure about the a6
b. adaptive headlight sensor. This is a little plastic arm by the front tires telling the computer how much weight is on your suspension
c. of course the headlight assembly itself with the motor moving the bulb up/down.
d. check the connector and make sure it's properly attached and has good power

* rear passenger window:
I. (single window) check the window regulator
II. (single window) check the window motor in the door
III. (single window) check the button - if you can still move the window by either pushing the window button in the rear or the master button by the driver console, it's just a switch replacement
IV. (both windows) - check electrical connections and convenience module (or also known as comfort control module) obviously there is a bigger problem going on than the above mentioned things if they both stopped working at the same time