have just replaced catalytic converter and manifold and now hearing whining noise while driving that seems to be coming from back of car ... why?

So I understand clearly. Did you have your cat converter and manifold replaced already and now its making the noise? Or do you need to have that work done still? If you have had it done there are many possibilities and a whining noise is very vague. You are going to need a hands on inspection to get a proper diagnosis. Here is a link to help you find a good shop near you. https://repairpal.com/repair-shops Alex