Transient high-pitched squeal in '02 Accord

Last night, while stopped at a traffic light, I heard a warbling, squealing
noise which seemed to come from under the hood. The closest thing I could
think of that sounds like it is the sound of the steel wheels of a rail
car screeching as the train comes to a halt. We were in traffic, and there
was no place to pull over, so I did what I could to figure it out. Putting
the car in neutral and racing the engine did not change the noise. After
about 5 minutes it disappeared (before I could stop anywhere) and did not
return while we drove for the rest of the night.

When it warms up later today I will check belts and fluid levels, and look
for leaks. After the fact, though, it occurred to me that it might be the
bearing of the heater/AC blower, which was running at the time and which
I did NOT think of playing with at the time, to see if the fan speed affected
the noise. Does this sound possible - could a worn bearing or dirt in the
shaft of the fan motor make such a transient noise?