My sorento has a very irritating 'tinkling' (for want of a better description) noise when cruising on a light throttle. Sounds a bit like a petrol engine 'pinking' when under load. It disappears if you lift off or accelerate. Seems to be coming from the engine but it is not the usual diesel knock, it has a much lighter tone. Any ideas? I also think the engine is noisier than any other on the market.