When I turn my hot water on in the bath, after a few seconds the pipes starts making a whiney, grindy noise (not the common hammer banging sound) What is it and what can I do to stop it. It doesn't continue the whole time I run the bath just intermittently - thanks

The oscillating sound your hearing is probably a loose washer or valve sealing material. Usually it stops when you move the dial to a different position. Change the cartridge, or rubber valve material.