[s-cars] got a new sound

[Mike Fitton]

And I'm at a total loss.  It's a very clean vibration.  Don't feel it, 
only hear it.  Frequency rises with ground speed.  Amplitude varies.  I 
don't hear it below about 20mph and not above about 65mph.  It's loudest 
around 50-55mph.

I've ruled out everything engine by turning the engine off at 55mph in 
neutral.  Stick and clutch position don't change anything.  Steering 
wheel doesn't change anything.  Wheel bearings all pass the 
autocrosser's tech inspection test.  All CV boots are tear-free as far 
as I can tell, and none of the usual grease sling that comes with torn 
boots.  No perceptible alignment changes.  No corollary bugs at all that 
I've noticed.

About two months ago, I changed out my transmission oil (finally).  
Amsoil's synthetic 75W90 GL4.  All the plugs are still tight and the 
stick still feels great.  The sound started very suddenly about a week 
ago, so I'm guessing it's not that.

Two weeks ago (one week before the sound started) I switched back to my 
summer tires.  Drove on those just fine for the first week.  Pressures 
are consistent.  Verified lug torque at 90 lb-ft for all 20.  Rotated 
them anyway with no change.  Tires and wheels all look fine, and no 
feeling of imbalance in the drivetrain.

All I can think of is the prop shaft center bearing.  I've never 
experienced that before, so I don't know how it should sound.  I played 
with my prop shaft (no Krausky, not like sexually; or at least not right 
then) and found a little front to back play which seems like it would be 
normal with a CV joint on either side.  The two halves seem to move 
equally during this exercise.  There's also a little play perpendicular 
to the shaft at the center - that is, I can bend it up/down/left/right a 
little at the center U joint.  I never did that before, so I don't know 
what's normal there.  But it seems to me like it shouldn't do that, so 
that's my best guess so far.

But before I give up on diagnosis and resort to just throwing parts at 
it, I want some second opinions.  Anyone?  Bueller?

-Cheers!
Mike