%60 of brake squealing problems come from not being shimed properly. After that, there are about 6 other things it could be. Boxster S's have 4 round ones (per side) that insert into the pistons on the back brakes. they should be glued to to the brake pads. These may have come disjoined. the fronts have (2 large rectangular shims per side). I just changed my brake pads today. I used brake disk quite, made by permatex when installing my new EBC Redstuff.
If it's not that it could be:
Rotor quality
driving style
pads improperly bed-in
glazed rotors
drag (from a seized piston)
Pads do not have Chamfer
Hopefully its your shims, its an easy fix. glue them back like they should be. My car had 60K on original pads. Fronts were just about to activate the replace pad sensors. I had to pry all of them off the shims.
good luck
