Persistent vibration, 87 5KQT

[John Larson]

I'm about at the end of my rope.  I have a customer's well worn, high 
mileage, 5KTQ here that has developed a low pitched humming vibration at 
freeway speeds, although the noise persists as low as 30MPH, maybe even 
lower.  I've checked the axle shafts for bends, replaced a bent RR 
wheel, and aligned the driveshaft with a cool home made fixture.  I 
spoke with a chassis and alignment guy this afternoon (he's gonna drive 
it Monday) and he asked about the wheel bearings.  I've done probably 
500 double row wheel bearings in my career, and I've never had one sound 
like this.  They mostly howl or make a low frequency rumble, and usually 
(but not always) change tone or volume when side loaded by turning the 
car one direction or the other.  This noise comes on as you build speed 
on the freeway, and is there whether you're going straight or turning.  
The volume, tone, and frequency appear to remain pretty constant.  As 
always, any and all constructive input would be sincerely appreciated!

TIA!  John