Pumping it up

I HAVE had my 1994 VR Holden Calais since new. Recently, it has developed a intermittent hammering sound when the steering wheel is turned at very low speed, for instance, when parking. The car has only been serviced by Holden dealers. One told me I needed a new power-steering pump, which was fitted without any effect. The second claimed the wrong pump had been fitted and told me to go back to the first dealer, who disagreed with the second dealer's diagnosis. I then tried an independent power-steering specialist who claimed a reconditioned pump had been fitted and that he would fit a genuine new pump, which would fix the problem. It didn't. No one seems to know what's causing the noise. Can you help?