When I'm braking at a stop sign, I'm hearing a sound that sounds like a high pitched tire squeal; it's like a mix between tire squeal and the constant chirp from a bad belt, and it's coming from the front of the car. I believe it's coming from both sides.

At first thought it seemed obvious that it was the tires, but here is my list of thoughts:

* Tires are high-end, nicely broken in, yet still have plenty of
tread, so maybe it's the brakes?
* On the other hand, the brake rotor have only the slightest (1mm, 2 at most) groove and the pads are still at least 1/4" and none of the sensors have come on.
* This only happens when the car is warmed up (which should make the tires grip even better, right?)
* I tried this in neutral and the noise still happens, so it's not the drivetrain
* This only happens just before coming to a stop, even with modest braking. I've slammed on the brakes when going fast and I don't get the sound
* I've heard it could be something stuck on the pad, but I don't see any strange grooves on the rotor
* The sound is only for a second

What could it be?