It is very rare for me to be outside the car with the engine running. Yesterday I started the car and hopped out to clear leaves from under the rear wiper blade. As soon as I got out of the car I could hear a very low pitched rythmic and barely audible deep knocking/thumping noise from the engine!! The sound went away when I revved the engine but it did sound worryingly like a big end knock. Since new in July 06 the car has only done 24k miles, doing mainly motorway miles by the wife who is very lightfooted. I drive it spiritedly at the weekends but don't thrash cars. I'm going to have another listen tonight and then look at getting the dealers to have a listen to it. I seem to remebera nother thread some time back about engine failures on the 170 which I was quite cyncial about at the time.