I have a 04.5 with 14000 miles, it has what I think to be a fuel knock, very noticable when at warm up temperatures and still noticable at normal operating temps, I have tried many different fuel brands as well as fuel conditioners, the shop tells me that the occasional 600 is noisier than others and not to worry, I live in a small community and wonder if the shop is up to speed on this issue,anyone else out there had this problem, if so any suggestions.
Thanks.

Ruhbarb, how do you pick out fuel knock from all the racket the engine makes, haha? You might get your garage to hook up the engine diagnostic computer and see if everything is ok. This would tell if engine is running ok and maybe ask them at the time if there is another 'program' to load on your truck. I think a recent one gets some people better mileage and possibly smoother operation in the cold weather. All of these engines make some noise.