My sons 08plt A3 quattro 39k miles, at approx 70mph developes a type of booming noise in the cabin, but drives through it.
It only sounds at this speed no matter whether in 5th or 6th and is not engine revs related in any of the gears and without any vibrations etc.

Any clues ????

thanks

 

check the tire wear --- could be feathering or some other form of uneven tire wear