It has to be either your wheels or the brake rotors.

 

This does not at all HAVE to do with the wheels and tires. If it was a wheel balance problem it would depend on speed and not whether or not you were accelerating or decelerating. If it was a brake rotor problem, it would only happen when you were applying the brakes or slightly afterward when the pads are still retracting back into the calipers.

With that out of the way, I've seen this on plenty of Allroads. I'm not saying that this is the case with you, but you may want to check it out. The Allroads I've personally seen with this same issue usually have bad inner CV joints and usually on the front. These same people that replace their front axles with aftermarket (non-audi) axles generally have the same problems in about a year or so because the aftermarket axles aren't as good as factory units. If you can feel any more than the slightest amount of play when wiggling your front axles up and down then the inner CV joints are probably bad. Just make sure that the output flange on your trans is not the place where the movement is coming from.

Also, the upper control arm bushings could cause this problem if you've already replaced the axles and it's still happening. If all else fails, you can see that your bushings are bad, you might as well replace them because you know that they're an issue that needs to be fixed.

 

Peter,

You were absolutely right! Three of my front upper control arms had cracked bushings so I replaced them. It was the main cause of vibration, especially under acceleration. I drove the car only for a day and no vibration so far. In addition, I just wanted to rule out any impact from front axles (made by Lemforder) which I replaced earlier. They were still under warranty so I replaced both of them once again.
FYI, I took my car to Audi dealership for diagnostics when this vibration first occurred. They issued me the following verdict: “Prop shaft is warped and needs replacement. Approximate cost of repairs is $2,400.00”. I’m glad that I didn’t follow their "professional" recommendations. Imagine spending almost 25 hundreds and still having this problem?

Guys,

Thank you for taking time to respond to my thread and for all your tremendous help!!!

I really appreciate it!

Problem is resolved, case closed.

 

As you can see from this thread replacing 3 front upper control arms fixed the problem. So far it’s been fine for more than year. But, I’ve been having another kind of intermittent vibration lately and again during acceleration only. It randomly happens when I drive up to hill or car shifts between the gears.

What could cause this kind of vibration?

Thanks in advance.

 

Although you said the problem went away with replacement of control arms with worn-out bushings (apparently needed) and new tires, all of this new rubber could have absorbed the vibration of another problem that never was addressed and made it less detectable or completely masked it for the time being.
All the rubber (arms) has had plenty time to settle in, along with significant tire wear (I assume) to have lost a lot of "cush" that would absorb vibration.

Auditech79's logic was spot on...vibration on acceleration can only be caused by a rotating mass...not just worn-out bushings on control arms (which would magnify even a small vibration), especially if bad tires were ruled out.
Rotating masses include the driveshaft, joints on that shaft, etc...
I'd return to the original diagnosis and maybe have a qualified mechanic recheck the central driveshaft (prop) and its joints/flanges (especially) and all other rotating parts....outside chance of transmission, but more likely prop shaft or even inner front drive joints...those are the areas where I'd look.

 

P.S. While on the subject: I NEVER use aftermarket drive axles. There's a reason why they cost a fraction of OEM's...that's ALL they're worth. Most people replace drive axles with these cheapo's simply because of ripped boots...NOT failure. I replace the boot and keep the original equipment. This may not be the easiest way to go, but it is the strongest and longest lasting.

When caught reasonable early, the boots are replaceable, with no damage whatever to the original CV's, which when kept clean and greased are basically bulletproof and last forever...unlike cheap, imprecise, crap metal imitations. The only time I've ever had an original CV start failing (severe torque steer) was on my first FWD (VW Rabbit), on which I stupidly decided to ignore the grease splattered all over the fender well and it still took thousands of miles to fail, wide open to the elements.

These axles/joints are one of the highest stress points in the entire car and deserve top quality, especially in a quality car with significant horsepower. You wouldn't use a dental crown made of paper machete.

 

I've "NEVER" fixed an audi replacing the prop shaft. I've diagnosed a couple of those myself with the shop forman backing me up, and we both looked like idiots when it was still broken. Also don't think that tire techs can balance a tire like its second nature, i had to go back to Les Schwab 4 times to get my tires balanced correctly on my Sierra, i eventually offered to "show" them how its done to fix it. That got me banned from the store after i fixed it the first time hahaha. I would go back to the wheels, brakes and perhaps front axles having torn boots.

You must have had some serious bushing damage on those upper links to cause a vibration like that, usually those only last 50k on the D3 platform. Primarilly because audi used the same bushings as the A4, but beefed up the links, one of the only things i have found on the D3 A8 that audi saved money on.

 

Guys,

I appreciate you taking your time to respond to my post. I'll mention all these possible causes to my mechanic.

Thank you very much again!