Does the squeaking stop if you gently pull the handbrake up a little? If so, the noise is caused by the rear pads rubbing on the discs, and tends to be more prevalent when the brakes have warmed up. This is usually due the pads sticking in the carrier due to rust on the edges of the pad backing. If the pads still have plenty of life in them, they can be removed and cleaned up, with a little copper grease applied to the edges on re-assembly. This is a very common problem; I often have to "strip and clean" the rear brakes on cars in for service. Other potential problems are seized caliper slider pins, and sticking calipers, as well as defective handbrake cables (sticking and not releasing correctly).