It never used to do this, but my S3 has for the last week started knocking from the front when I brake lightly. Sometimes this does not occur until the pitch of the road changes again and the faster I go the quicker the series of knocks (usually around 2 to 5 seperate knocks) when I brake. These knocks can be felt through the brake pedal as well.
Taking the car along the highway earlier and cruising above 100km/h I can actually feel something touching somewhere lightly, again through the brake pedal and there is definitely a metallic friction noise somewhere. The front pads and discs were replaced a month ago.