I may have been mistaken about the 'singing' noise in my '93 Sentra.
Simply rolling down the driver's side window has added a new dimension
to the problem.  Let me know what you think of this:

At low speed -- 20 miles per hour or less -- there is definitely a
metallic "swashing" noise. By that, I mean it sounds like a steel brush
scraping sheet metal with each turn of the wheel or axle. It's not
always there, but when it is, it sounds like hell.

I thought I was lucky to get the axles replaced a week ago for $275 (my
parts), but now I wonder if I should even go back to that mechanic to
have him check out the noise. Originally he quoted $650 to replace axles
(supplied), thermostat (supplied), and two rear wheel cylinders, change
the fluids, and do... eh... something to the transmission (5-speed).
Can't remember.  Basically a 50,000 mile maintenance.   Yes, it has only
52,000 miles on it.

I balked at the cost, but the axles were an emergency repair, so he
charged $275 for labor to replace them and do an alignment. He also said
the thermostat job required a "special tool," so he couldn't do that. I
have not had the chance to even get under the car to see if something's
wrapped around the axle.

Ratcheting noise from the failing CV joints wasn't as bad as the noises
it has now. I.E., the Sentra he returned to me has more noises than the
car I gave him. Do you suppose the 'special tool' he mentioned might be
one used to screw a customer who can't/won't fork over $650 all at once?

I apologize if I sound meanly suspicious, but I can't believe that he
could get this result on a job many amateurs do easily.