Originally Posted by zze86

In those gears, at those RPMs, you're doing 60+ mph which leads me to think it's a balancing issue. Since you've tried different wheels with no effects on the vibration and assuming both wheel sets were properly balanced, the next most likely culprit are the wheel bearings. Any droning or whining occurring at all?

You can check for play and see if there is any excessive movement around the hub. Lift up the car and wiggle the wheels around with your hand at the 3 and 9 o'clock position as well as at the 12 and 6 o'clock position. The wheel itself should not move.

You could also try having someone spin the wheel while you listen with a stethoscope around the hub for weird noises but since you're encountering vibration at high speeds only, you may not hear anything.

Unfortunately, the wheel bearings/hub in the S are not easily removed/taken apart for inspection so this is about the extent of diagnosis before replacement.