could be awheel speed sensor or a displaced abs ring? Its weird but my mk4 started making this gringding on lock and when low speed braking, and it was the wheel speed sensor exploded! abs packed ina few days later, but it was weird cos it jsut kept releasing the brakes, thinking it was locking up, even though it wasn't! So The car would not stop in traffic and I needed to stop wiht the handbrake!!!
if it is this and you loose the brakes, take the abs fuses out that are on top of the battery and the brakes comoe back with no abs though!!
(good tip for trackdays too!!)