R.E. Mixed tires on Quattro?

[mark.e.woodland at gsk.com]

My limitted experience revealed that it would tolerate the new tires on the
rear, but when they were on the front for a few days, the car exhibited a
strange rythmic vibration cycle.. 15-20 seconds of a subtle vibration,
followed by 15-20 seconds of smooth ride. It stopped as soon as I rotated
the new tires to the rear.
The theory at the time was that the center diff was coming into play, as it
sensed speed differential between the fronts and rears as tire slip.  I
upgraded to all 4 new within a week, anyway, and its like having a new
car..