1997 Jaguar XJ6 Engine rattle - Timing chain or Steering pu

I have a rattle coming from the engine bay only appears once the engine is warmed up and is worse at about 2000 rpm. My mechanic has diagnosed it as timing chain, however, the noise disappears if I turn the steering wheel with the foot brake applied. My feeling is that it is related to the power steering pump. How can I be sure and if it is the steering pump what can be done to fix it?