Thanks for the reply thomas, the sound you describe is pretty much spot on. It is intermittent, depends on the road surface, usually kicks in over 50mph.
Ive developed a strange noise at the front end of my Audi A6 C7 2.0tdi, it comes and goes and doesn't sound like a wheel bearing more of a hollow wind type noise, anyone had any similar experiences? (Apologies for the noise description as I find it hard to describe)