I've got a '52 Mondeo TDCi (115PS) and it makes a slight whining noise. It's directly related to the engine revs, and it sounds a little bit like reverse gear but maybe slightly harsher, and quieter. You don't really notice it from inside, only with the windows open or when standing next to it, at low to medium revs.
I think I've heard other Mondeo TDCis make this noise, and I know I've heard it from an '04 Peugeot 307 HDi, but I don't know whether that's the same type of engine. Is this normal and harmless, or does it indicate that something expensive is likely to fail soon? I already know the flywheel needs replacing, but from the nature of the sound I would be more likely to guess it's a bearing in one of the ancillary components eg PAS pump. If there's likely to be another big repair bill soon I don't think it'll be worth doing the flywheel.