i would have the surfaces the discs go on to checked for rust or dirt or maybe warped,, ie dial gauge and rotate them, steel discs can have a habit of getting very hot and when cool with brake pads touching can also change the make up of the metal ie distort the discs, also a worn out brake caliper that holds on a bit will generate heat more and will also give some noises most of the time,
i had a new 3 series bmw back in 2004 and i drove it very hard and that i could ruin brake discs within a few months if i over used brakes ie pushed it past a certain limit,, i ended up replacing the discs and pads with AP RACING and never had the problem again, i believe that AP racing heat treated them before and after machining the faces on them and were drilled and grooved and the pads were a compound that just stopped the car so much better faster,
