I've read that BMW brakes don't last as long as typical American Farm Machinery (ford, chevy, etc), and it used to be that if you heard squealing, it meant one of two things, either their was dirt, debris lodged inbetween the pad and the rotor, or that the pad was worn down to the "Attention" metal strip imbedded into the shoe that emits a squealing noise telling you its time to change them. I'd pull a wheel off and take a look at your pads, see how far down they've worn.