I recently bought a used 97 A4 1.8T Quattro and when I'm driving it there is a very loud, loud humming noise coming from the rear. (The faster I drive, the louder the noise gets. At about 70 MPH the noise is so loud the radio needs to be at full volume to even hear the music) I assume this is because of the 'quattro' system but it is VERY loud and i'm wondering what could be causing this? Does something need to be lubed up; new bearings; or new something?? Any help would be appreciated greatly!