🚀 go-pugleaf

I have what I think is a rather unique brake noise to deal with.  Everytime
the speed of my car (an '84 Plymouth Horizon) drops down to around 5mph or
less, I get a fairly quiet high-pitched squeaking sound from the left front
wheel, sounding pretty much like air leaking slowly from a balloon.  What's
interesting is that the noise is exactly the same whether or not the brakes
are being applied at the time.

I followed the factory shop manual's instructions for similar squeaks, which
was to clean and grease the caliper adaptor guide rails (what the pads slide
on) and re-glue the noise suppression gaskets (which were still sticking fine)
behind each pad.  This got rid of the noise for about 1 mile, then it returned,
sounding exactly the same as before.

Rotating the wheel in question produces a very slight dragging noise from the
disk, but no more so than for the other front wheel (which doesn't squeak).
Also, the caliper remains cool to the touch after a long freeway run, and
there's no sign of excessive pad wear or overheating, so I don't think it's
dragging very much.

It's only been doing this for a few days, but I'm something of a fanatic
about unidentified noises from my car and this one's driving me crazy.
Does anyone out there know what this is or how to determine it?  Will it
likely go away on it's own after a while?  I hate taking the car to the
dealer, but I'm just about ready to try it.  Any suggestions, by net or
E-mail would be much appreciated.

P.S. Further relevant info: Car has 28,000 mi, gets almost all freeway driving
     (brakes are hardly worn), and the brakes function perfectly otherwise.