Vibration - Ideas?

Putting this out there for ideas basically.
I have an intermitent vibration when I get to about 70 - 80. (on a private road of course)

FEELS like wheel balancing.. but had them done recently ( I guess I could have bashed a pothole but don't think thats the cause)
Just replaced both front discs - which got rid of the braking vibration and I THOUGHT would get rid of the driving vibe - but on the motorway, its still there.
What could be causing it outside of Wheel Balance and new discs? Seems to be worse under load, i.e. accelerating up a long motorway drag, as opposed to coasting where it doesn't seem bad at all. Engine revs freely.. no weird vibes or rattles from that.. am stumped!?

[Conan_the_Librarian]

Any vibration when revving the engine in neutral?
If not then it's probably a rotating mass issue, so with new disks and pads and recently balanced wheels, that leaves Wheel Bearings, CV joints and half shafts. I gonna plumb for CV joints wearing out. Have you ever had to replace split CV boots?

Definately something rotating. Engine revs freely and smoothly.. have tried revving engine when stationery - lovely.. and also putting the car in Nuetral when coasting and freely revving - that also makes no change. I feel the vibe still there, but not as bad as when the car is pulling.
Might plumb for a quick rebalance incase I have messed something up.. but then look into CV joints.
Haven't replaced any split boots, and all looked well when changing the discs last weekend..

small update on this. Had another look around the wheel wells to see what I could see.. nothing obvious. No loose joints, doesn't seem any movement on the wheel. CV joints don't click to they too seem ok.
Went to the garage and thought I would inflate the tyres a little firmer than normal - and that seems to have made an improvement to be honest. Now not as pronounced and smoother at the aformentioned speeds. So that leads me to balancing then I guess.. (why else would the vibration reduce by upping the typre pressure!?)

one question - I have a set of standard 18" sprt wheels with 245 tyres on them. what is the best tyre pressure. At the moment I have them in my opinion quite hard up around 36psi, which seems to have reduced the vibe (although clearly I need to go get them balanced and thats not a fix!)

[PsYcHe]

I'm running mine at 40psi as I do a lot of motorway runs and that seems to keep them happy. I found I got vibration if they dropped to around the mid-30's. Nothing serious, just enought that I noticed.

Might vary from tyre to tyre, but seems to be the same for my original sport wheels with Dunlops and the replcias with Runways (cheap, but effective).

oh.. well I am probably a bit soft then (!) - I'll run 40 and see what its like. I always sit on the M'way too, with about 5 miles each end to Home/Work.
Defo seems a little happier being at higher pressures, just wasn't sure what the max / recommended is.

I have the same problem and now that you say something about tire pressure i noticed ,also that when they were around 40 psi the problem didn't feel as bad
I have had my tires balance at least 3 times every way possible but it had no noticeable effect . Mine when the vibe is present I notice a slight hum that sound like its coming just aft the cabin under the car like a drive shaft bearing or U joint but since this is my 1st audi I'm not to familiar with the quauttro Drive train but I'm learning
Keep me posted if you find out anything sounds like we have a very similiar problem mines starts its vibe at around 65-70 and plains out just a little over 80
Greg

Thats the one my friend.. I only have 2 wheel drive, but that exact symptom. I think now the tyres are way hard, its actually a little worse then how I got it by inflating just a little more than normal. Buggered if I know what it is, get a vide about 70-80 (worst place) low pitch hum around 85, and can make it worse applying power, or make it better by letting off the gas.
I am really hoping its not diff related like SKA had once on similar car..

Update Story part II
Ok, so yesterday was MOT day on the car. Still got the vibration.. hoping its not some ball joints or something that are going to fail it but otherwise confident the old girl will go through ok.
The wife had the day off, so I set her the task of getting the MOT done, handed over £30 (Citroen Garage in Morecambe does £29.99 MOT's) and I bimbled off to work in her polo (reminding me how good the 8 is, even when its bad :-)!
About 1.30 she calls me up to tell me all is well obviously.. and .. its failed.
Front NS tyre, bald practically around the inside. had no idea - since I had the wheels balanced about a month ago, and new track rod ends fitted so I know the tracking is not out - but sure as eggs is eggs the tyre isn't legal. One can only assume it knocked the tyre out when the last set of 'new' track rod ends failed, and was just about ok when I had new ends fitted.
Anyway - I gave the other half my authority to empty my bank account.. and had 2 new Hankooks put on the front. Not the cheapest, but not the most expensive and to be fair the garage did a good deal on them. The car then duly qualified for a free re-test and passed with flying colours.
Firstly, this was frustrating, as I know stealers, and i know that women alone do sometimes shockingly get taken advantage of in car repair situations. I have experience of a Ford main dealer not replacing things they said they have when I checked once many moons ago, so having been initially dubious and frankly ****ed off that I had to shell out for tyres, I decided well they weren't ever new on the rims I got for the car, and this is safety we are talking about, live with it.
Now the point of all this waffle, is that I am thinking as I set off for work this morning in the 8 that its not going to make alot of difference really.. since I had the wheels balanced before and still had this worsening 70mph vibration.
Wrong. Vibration gone, car like driving on glass again at all speeds. How happy am I!
The moral of the story reminds me of my I.T. side of life and the one rule for diagnosing networking errors -always check the physical layer first. Those with IT heads on will know what I am talking about - that before getting too involved in stripping things apart, checking electronic systems that are probably fine and changing heaps of parts - check the simple things first. I always knew it was something going around out of balance to be fair.. but had ruled out wheel balancing, but should have maybe gone back to check first. You may recall my very noisy rear wheel bearings when I got the car.. that turned out to be tyres and totally dissappeared once I replaced the rears with the new rims.
So, happy again, just in time for my M6 south journey this evening for 3 hrs, and in the words of Gordon Ramsay,
MOT.. done. Vibration.. Gone.

[Architex_mA8tey]

nice one simon always satisfying when you get to the bottom of these things. A few years ago I couldnt get to the bottom of a vibration under a Limo that I was running and it turned out to be tyres just the same but after a lot of other expense was gone through first, so I echo that one, check your tyres first and even change them around to see if there's a difference!