I have owned a series of S-Class Mercedes. At least ten in a row, all wonderful, and all petrol. Recently, against my better judgement, I bought an ex-demonstrator diesel: a S320 CDI LWB, with 9,000 miles on the clock. It's not as smooth as the petrol ones but I can live with that because its fuel consumption is so much better. It's a lovely car but it has one fault/characteristic that is driving me mad. When it starts from cold it makes a metallic whirring or chattering sound (best described as like a cage full of budgies). It's pronounced and though it very much subsides when the engine is warmed up, perhaps after half a mile or so, I can still hear it faintly. All the time. It seems to be when the engine is under light load. The local Mercedes dealer has been endlessly patient but even the workshop foreman reckons he can't hear it. He has tried it from stone cold, and I have also been out with him in the car to reproduce it but he still is at a loss. I can't believe Mercedes send out S classes sounding like this. I know that diesels have an underlying growl that petrol doesn't have (our E Class is petrol and is quieter overall) but this just sounds wrong. What do you think it is? And apart from chopping the car in against a petrol version (a ruinously expensive step), what can I do?