I have a 2.5L diesel, 60 plate transit van which has a noisy engine on cold starts - sounds a bit like tappets. Once the engine warms up it quietens down. no loss of oil or power and the dealer has taken a look at it a couple of times and says it's nothing more than diesel knock! Have had the van from new and didn't have the problem in the first 12 months of ownership. Is it a problem or is this normal?