Get your head in near the wheel and see what the pad thickness is like, have you had them done recently or had the brake pads out for any reason? Sometimes if you dont use a bit of copper slip on the pads it can cause a squeel.
Just to show you (rubbish picture i know)
Look at the caliper near the disc, you will see the pad (black) if you have a good look inside your wheel you will see the pad.