do you get this problem when braking from high speed or low? does the steering wheel judder slightly in your hands? Do you get the noise when going round sharp corners (whilst NOT braking)?
I had a similar problem; the car would SCREAM when I braked from slow speed and would judder the wheel like mad when slowing from high speed. The car stayed in a straight line but it scared the hell out of any passengers!!

It would also make a noise when I went round tight corners (must have brought the pads back into contact with the disc through centrifugal force as the brakes were not being applied)
New discs sorted it.
If you are getting the noise without the judder then there is a good chance that your pads have worn down to the 'indicator'; it has been designed to make nasty noises when you're running short of friction material!