"You hear this noise just before the car comes to a complete stop."

You also stated this only happens when you;ve been in stop-and-go traffic, which tells me it happens when the brakes are warmed up.

I think what you describe is annoying but perfectly harmless. When your brakes are heated up from use, at low rotor speeds and only light to moderate brake pedal pressure, the friction linings are doing a "stick/slip" action on the rotors. The pads, which are not a tight fit on their locating guides, move back and forth in their clearances (not left to right, but relative to the direction of rotor rotation). This results in vibration which you perceive as the noise you describe.

I had the exact same problem on my 850 - I'd get a "graunch" just before the car came to a complete stop. I tore into the brakes to find the problem but found nothing. Careful experimentation showed I could make the noise appear pretty much on demand except when the brakes were cold (first or second stop of the day) or hot from some hard use.

Absent any pulsation or vibration from rotor runout or deposits ("warping" may be a myth, according to some brake experts), the noise is *just* annoying and is not indicative of problems with the brakes.

If your rotors are fairly new (like under a couple thousand miles), you might try a set of premium/heavy duty pads up front. I think I used Axxis Metal Masters which minimized the problem, but more reliable was to do about 3 or 4 hard stops a week from 60 or 70 mph, to get the brakes hot. You may not want to put up with that kind of regimen. Basically, the source of the problem comes down to the coefficient of friction between the pads and the rotors. This will vary with the type of pad used, and of course with the temperature at the friction interface.
--
In God We Trust. Everyone else, keep your hands where I can see them.