steering pump noise even after pump replaced

VW jetta 1995. 2.0 L, 155000 miles.

I am getting a hissing noise from the front rigth of the car. It only does it when the car gets hot (after a few minutes of driving) and the noise gets louder as I rev up the engine.

I took it to the shop and the mechanic said it is the steering pump making noise. First thing he tried was flushing the pump. He did the flush and asked me to drive the car for a couple of wees. The noise did not dissapear.

Then, he said we should replace the pump. He replaced it once and he noticed the noise was still there, so he replaced it again and said it was fine. I picked up the car, drove it for 10 mins, the noise seemed to be gone and the it reappeared!.

Now he says I should give the new pump a couple of days.

Is he on the rigth track? What could be the cause of this problem?

Thanks!