Odd noise (brake?) on 97 Taurus

97 Taurus GL sedan, 3.0liter Vulcan, 4 wheel disc brakes. ~120,000miles.
Recent work done:
Both front and rear brakes (pads and rotors, caliper service) changed about
3000 miles ago. OEM pads and rotors. Quality winter tires installed about 3 weeks ago (Pirelli Winter Carving directional snows, not studded)
Recently, I was driving at about 25mph, and heard a quiet mid-pitched whine sound (about the pitch of a car horn), speed dependant (the tone didn't change, but the frequency of the howl did (IE instead of noise-----noise-----noise, when fast it sounded like noise-noise-noise)). I didn't take much notice at the time, but the next day it made the same noise, only -much- louder. It did this once more (also loud), and then stopped completely. The sound seems to be coming from the rear drivers side wheel, and is very intermittent (3 times in ~250 miles)
I'm pretty sure it isn't tire related (it's -really- loud, and I can't replicate the noise just by driving at certain speeds, etc. It's very intermittent)
This is the car my wife drives fairly often, so I don't want to take any chances. Any ideas what I should be looking for?
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check the pads and rotors quick
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ok will do.. For breakage/cracking?

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Teknical opined in

for a cocked, or stuck, caliper
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Gotcha. Thanks again.
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It very well could be a tire making noise. It can happen on just certain types of road. Is there anything you can do to change the noise like turn or hit the brakes a little?
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