How old is the car, how many miles?
It could be a wheel bearing problem, however, I'd be more inclined to strip the rear caliper slides, clean and regrease with some appropriate rubber lubricant (either the genuine OEM stuff - G 052 150 A2, or sommat like Lockheed Rubberlube).
Also, remove discs, thoroughly clean mounting face on hub and inside of disc, apply a thin coat of anti-seize paste (ideally nickel based - but copper will suffice). Finally, make sure pad back plates and hammer heads are also clean lubed with anti-seize.
Do all that - and your brakes will be quieter than a dead nuns chuff!