I'd welcome views on where the grain is and if someone could explain the difference between grain and noise, or are they one and the same thing?
I've had a look again at the image and thought the grain may be in both the engines and on the tyres? The picture was taken in superb light, early morning (0950), with a 30D and an L series lens at 100 ISO.
I loaded it at 1600 as it almost required no crop and it looked as though the quality was there (got that wrong then).
I have taken the attitude on this site to take these rejections constructively and learn. I'm no expert but it seems that if you reveal too much detail in the shadow areas noise appears - perhaps someone would be kind enough to confirm that. To reduce the appearence of the grain is it better to darken the areas with say a levels mask, or use the noise reduction filter?