David Allroad

Hello, hope someone can shed a bit of light on this for me, I have a Allroad A6 3.0tdi sport quattro with 74000:mile on it, my car make a noise like pinking ,the noise you get in petrol engines when fuel is wrong for example. I only does it when I accelerate i.e underload not so much in the mornings but only slight, this seems to be the only problem and it doesn't effect the cars performance, and no fault codes, any ideas anyone.