Hello everyone
I am a new member. I have a 2002 C5 A6 2.7t quattro that i have had since new. I have done a few mods over the years (stereo, paint, lights, ect), but have kept drive train and tires stock, as i love the ride and it has more than enough power. I have serviced it religiously per Audi specifications. It has aprox 140,000 miles, and other than typical service, i have never had a problem. Until recently.... went on a 300 mile road trip, and noticed between 75-80 mph i am starting to get a shake/vibration coming from the front passenger side. It only happens in this range. I have had the stock 17" BBS rims and new tires rebalanced, and still have issue. i have had the axles replaced at 100,000 miles, and brakes with new rotors done about 3months ago @126,000. In town the car runs smooth as glass, and even when it starts to shake on freeway at stated speeds, i here no bearing howl and there is no abnormal wear on my tires. Finally, it only seems to start after a lengthy ride (after about an hour). Any suggestions would be greatly appreciated, so i can have some info before i take it to the shop.

 

Good luck with that. I have also a 2002 A6 but the 4.2. Same issue as you describe. It shakes annoyingly above 65 until almost 80 .. before and after those speeds it is butter smooth and drives great.
I have replaced: Axles, tires, rims, control arms (all of them), struts, brakes. Nothing has made it go away. It has changed over the years of replacing stuff but it is still there.
Most recent advise was to replace the sway-bar (+bushings!). Not sure if that will help but at this point I'm out of ideas and will give that a try.

Good luck and let us know if you find the reason.

 

+1 on the sway bar bushings weve got a fleet of bucket trucks and went thru rotors bearings ect... turns out it was the sway bar endlinks. Could be control arm bushings as well since the spindle is held in alignment by four bars rather than the more conventional upper and lower like a lot of cars so if one bushing gets loose its amplified. Post the results!

 

Is the vibration only in that speed range? If so, my bet is still tire (im)balance, especially if only on one side. I'm not sure how you can determine it is only one side at that speed. I've had the same thing on and off over the years on my 2.7T (231k), and usually new tires cured it...for a while. It is only a gentle vibration and I can ignore it although it, too, usually occurs in that sweet spot of high speed driving, 77-78 mph. So sometimes I actually drive the speed limit, or go like heck, and it goes away. If you find a cure, let us know.