2002 Volkswagen Passat inquiry

I recently bought this car. I noticed that when I turn the steering wheel in tight spaces I hear a rattling noise. It doesnt do it when im driving and turning. I took it to a mechanic. He suspected the steering belt. He changed it. But the problem remained the same. I took it back. He said he checked everything! But he could not pinpoint what causes the noise. He said it could either be the steering pump npt giving enough air ot the streering rachet. He advised me to take it to the vw dealer. Please help.
thank you very much
hafid.