is the sound always there?(car running but not moving)
If so turn the wheels and see if the sound changes. if it does then its most likely you p/s pump which needs 2 be replaced if the systems is already topped up.
If the pump runs low on fluid then the internals will wear and then keep making noises.
If the sound is only when driving then there might be a problem with bearings. but i think its just the p/s pump