New car... Strange noise

[HudsonNo1]

Hi all, I've just brought a mondeo 2.0 tdci obviously a used car but beforehand I've inly ever had a little 1.4 fiesta, the mondeo is making a kind of low grunting noise on acceleration, it doesn't last very long and isn't for the whole duration whilst I'm accelerating... This may be totally normal but as I've never had a car like this I'm not used to the noise... Wondering if anyone could either put my mind at rest or point me in the right direction of what a problem could be..

Thanks in advance