klouisdrk

If anyone on the forum has experienced this, please help me to find the cause. 2006 A8L 97k mileage. I'm experiencing a thumping vibration when I reach 40 mph and gets progressively worse the more the speed increases. I don't put a lot of yearly miles on it. so I replaced my tires recently with an alignment because the previous set were 6 yrs old. I thought that would solve the problem (it didn't). From past experience I know wheel bearings can also cause vibration so I replaced them also (didn't help). I'm stumped, If anyone else experienced this and found the cause, please share. I hope someone here can help me to track down the issue.

TSHong

Eliminating tires would be the first step and typically, bearings make noise but not a vibration unless they are really bad. It is something that rotates so the next thing I would check are your cv axles.

dvs_dave

Where are you feeling it coming from? Front, rear, left, right, middle, under you, pedals, steering? Does it change with vehicle speed or engine speed? Does it change when turning?

Initial guess is a CV joint or a prop shaft UJ or support bearing failure.

klouisdrk

It's kind hard to tell, the wheels seem to be rolling smoothly, The steering is true, under 40 mph I feel nothing but smooth sailing, the thumping vibration starts subtly at around 45 mph and get progressively worse as the speed increases, has no effect while turning, it feels like it's coming from the front middle of the car if that makes sense. And by Prop Shaft are talking about the actual Drive Shaft.

MP4.2+6.0

Start w/ tires. Since recently changed, check wheels for bends and flat spots. Wheel shops can miss flat spots if they just close clover, spin wheels up and robot attach weight but no pay attention to actually looking at it or spinning by hand.

Per others, after that something like main. prop shaft. On my C5 4.2, at higher miles and when older (as in, D's now...), I found multiple flat spotted wheels (factory OE). Separately I had a torn boot at back or the Torsen mid diff at front of the main shaft.

MP4.2+6.0

One other "never say never" idea is check the wheel bolts for proper torque. In 50 years I did made the mistake once, and saw it one other time after "shop" work. Just had it recently with Bozo of CA AAA of all things who brought his $20 battery impact wrench for a road side spare/flat change.

Likewise check if you happent have NON-hubcrentric spacers. If so, likely remove them (and be sure lug bolt length is stock).

As these cars age out, more and more non-stock and who knows what after 3 to 6 owners.

klouisdrk

Thank you for trying to help. Is there another name for the "Prop Shaft"? I tryied to find that on an Audi parts site by that name and didn't get any results.

aTOMic

I had to replace mine and I seem to recall the dealer wanted ~$1400, but I found it for $500 on this site; I'd make darned sure it's the correct part before buying, though. Also shipping would be a killer; it's huge.

aTOMic

I ended up getting VERY lucky and found one 20 miles away in a parts yard for $50. Bentley says it needs to be aligned with an official Audi tool that's as long as the shaft itself (unobtanium)... I used a laser level from several angles and it's been fine for (I'm guessing) 20K miles! Hardest part was removing all the allen head bolts which are VERY tight for their size; the shaft turns while trying to unscrew 'em and I could not figure a way to clamp it... I bought a cheap but humongous impact tool from Hazard Fraught which worked like a charm.

klouisdrk

Thanks for the info. Is the drive shaft the part you replaced? And is that also called the Prop Shaft or are they two different parts?