Folks, I have a 1991 V8Q with 205K on the car, and less that 50K on the engine and transmission. Starting at 65Mph to up to about 80 Mph the car has developed a nasty vibration while under power and the wheels are held straight, the vibration stops when I turn even slightly to the right or left, and resumes when the wheels run straight. Lessens dramatically when I let up off the gas, even tho speed is constant. Any ideas ?
I tried the suggestion of the tight turns, and there's no noise whatsover, the cv joints and differentials are quiet and smoot. But that would only really flex the outer cv joint, maybe the problem is with the inner ????

my best guess would be CV joints but all of the abouve could be the cause. as well as something as simple as loose lugs or if you have after market rims with out a centering ring your rims could be slightly off center. Most probably cv joints. you can check for bad CV by driving in a tight circle in a parking lot and listen for clicking. this could be hard with an older quattro as there is some noise from the differentials in tight turns which you would have to be able to distinguish between.

cv joint...wheels need balance,,,bad tie rod ends/ball joints..bad struts...with that many miles the whole front end is suspect.