C15 1.9 Diesel Strange Noise

[c15]

Hi

I'm scratching my head trying to think what could have cause the following: I was driving along when all of a sudden my engine started making a strange noise a bit like a blowing exhaust, which was noiser when I put my foot on the accelerator. On stopping and opening the bonnet the engine was making a loud knocking noise and vibrating. After switching off the engine and restarting everything was fine with the engine running sweetly again! It was as though nothing had ever happened. The engine has only covered 20,000 miles and has performed perfectly to date. Any ideas anyone might have as to the cause of this would be gratefully received.