2003 Tiburon random metal on metal grinding front brakes. Pads, rotars and calipers all good.

Hello I have a 2003 Tiburon and I recently
had the brake lines replaced for the rear
along with new pads and rotors etc. When I
brought the car to the shop I explained to
them that it would randomly make a
grinding noise when braking. This would
usually occur when I did a lot of starting
and stopping in the city at various traffic
lights. After the rear brake line blew out, I
drove the vehicle, as it was only a few
blocks away, rather than having it towed to
the shop. During that short trip of course
the front brakes were still in working
condition but every time I applied the
brakes I got the same grinding sound from
the front brakes. Inspection at the shop
showed that the brake pads the rotors and
the calipers on the front brakes were all
working properly so we are stumped as to
where this metal on metal grinding sound is
originating. It is again a random occurance
now that, at times, lasts an entire trip only
when applying the break.  Any thoughts or
help on this matter is greatly appreciated. -
William