AudiForums.com - Annoying vibration under center dash

Just brought home a new-to-me '05 A8L w/ 36k miles and the car is everything I thought it would be and impresses me constantly....it's like a private jet for the road. The last two days though, I have noticed an annoying vibration under/inside the center of the dash (at least that's where it sound like) and it only does it over coarse sections of road....it's almost a harmonic sort of deal w/ only certain sections of road that can make it vibrate. I have tried pounding/punching/touching/holding/grabbing all different dash panels and pieces to try to zero in on the location/cause of the vibration but I think it is inside the dash somewhere. Has anyone else had this problem? Any ideas on a (haha)quick and (lol)easy fix?