[s-cars] bearing rumble

[Jerry Scott]

I saw an earlier post that had the same problem as my car, but can't 
find it, so here goes.  This is definitely a front of car, drive line 
noise and not engine related.  Normal 60 mph driving produces a low 
frequency bearing rumble that sounds almost like a tread buzz on 
aggressive snow tires.  Since my tires are m+s with a very small tread 
block, I know it's not the tires.  It has gotten progressively worse 
during the last 3000 miles and is aggravated by turning to the the wheel 
to the right at road speed.  Turning to the right seems to make it more 
pronounced.  I have checked the boots on all four front CV joints, which 
look fine with no tears or leaks.  Jacking up the front wheels and 
spinning the tires does not produce any noticeable axle bearing or CV 
joint clicking.  My first thought was that it was a wheel bearing going, 
but laterally pulling on the front wheels shows no play or roughness, at 
all.  The sound seems too low to be the usual trany main shaft bearing, 
but could it possibly be the  output axle bearings for the trany?  It 
also could be the center drive shaft bearing, but sound more like it is 
in the front of the car rather than mid-car.  Any advice would be 
appreciated.
Jerry Scott
early 93 S4