screeching sound

hi I have 1994 volvo 850 , I have a screeching sound in the front corner while the car is runing. the sound it constant and does not really change while the car rev's up. The Mechanic changed the power steering pump and the noise is still there. He now wants to change the power steering rack . I do not think the noise could be coming from the rack. Car drives fine,exect for this noise. Any advise or help would be great .