On some tunes one of the pilot injection events turns off around then. It really clattered on mine when this happened.I like that the 6.4 engine noise is quieter than the older models but my 08 gets really loud at around 2250rpm. I'm not sure what it is but I know my 2010 didn't do it. Is there something wrong or is it normal?
check for work rocker arms or broken rocker clips.
could also be injector rattle.