Hello. I have V6 Tdi s tronic. When I excelrate there is a tinny rattling sound. It reminds of the old days when thr air file mixture was wrong for a petrol car. Please can you give me any ideas why it would be doing this

It's the same problem that you remember from the old days from your description: pinking or pre-ignition. It's the result of too weak a fuel mixture or a spark that's too far advanced or fuel of too low an octane rating. If you are using the right grade of fuel there is a problem with the engine that needs diagnosis.

Thank you for your Advice. Off the top of your head do you know what octane level the diesel should be? 

Diesel? You didn't mention diesel until now so I wrongly assumed the fuel was petrol.

Diesel does not suffer from pre-ignition so your problem lies elsewhere. Incidentally diesel, being a longer molecule, is measured by decane rating. As far as I know only one grade of diesel is available here, except for the anti-waxing additives in the winter.

Thanks sorry I didn't say it's diesel. 

Guess it's a diagnosis I will have to book it in for