could be a sticking caliper.
could be brake pads. if there pretty new and they are low then have a look at your rear axel bushes, if there worn it can give play in the axel in turn causing the pads to rub against your breaks.
mine used to squeel every so oftern going forwards but most of the time going backwards because the rear bushes were worn and had made one of my pads wear down and a small peice of metal was rubbing against the disk.