I have just changed the front disks and pads on a 2002 Volkswagen 2.0-litre petrol Passat. All the bolts and guide pins on the caliper and clamp are tight and torqued correctly, as are the wheel bolts. The brakes seem to be working fine and are stopping the car properly, but when travelling slowly and applying slight pressure to the brakes there is a 'clunking' noise that seems to come from each front wheel. It only happens once, each time the pedal is pressed. I've had them apart a few times now to see if I've missed anything but can't find anything out of order. Any ideas?