If it's a front axle, I would think you should feel it in the steering wheel. Or if it's wheel balance related you could feel it coming from the corner(s) of the car, similar to having snow packed wheels. If it's a resonation from the center of the car, like under the center console, it's most likely propshaft related. My vibration, like you described, was caused by a bad front CV joint on the propshaft. Could also be a misaligned propshaft if you've had related work done or a failed center support bearing.

Another user had these symptoms recently but I haven't seen any follow-up. There are few suggestions in that thread.

http://www.rs6.com/showthread.php/33...l=1#post288192

Engine mounts would throw a code and vibration would fluctuate with rpm. Diff mounts would be an on/off clunk when the drivetrain is loaded/unloaded. Braking related could be felt in the brake pedal (warped rotor).