Citroen C3 screeching noise when car gets warm

i have recently brought the above car and it's great but after driving for a while when the car warms up I get a screeching noise and when I put my foot on the brake it goes and then comes back again my mechanic thinks it could be the servo unit and its an expensive job it seems that it's one problem after another as had to have new part for central locking. Please help many thanks mandy